I suggest that whenever you begin to use a new technology that you start by getting familiar with what the presets can do and HOW they do it.

One way to do that is to read and try the 'Mastering MODX: Rhythm Pattern article. Is shows you step by step how to use a preset performance, add a drum part and then play along.

If I have a performance with a piano and drums and create my own drum pattern using the Pattern Sequencer and then play it back then whichever keys I used to play those drum parts also play the piano.

Yes - that is what will happen if you just mix two parts that haven't been designed to work together.

Every part/element has parameters that determine WHEN it plays. The basic parameters are the key range (used for splits) and the velocity range.

If you press a key manually then every part/element that has that key in its key range and has the velocity in its velocity range will respond. So if both your lead part and drum part match then both will sound.

Since the drum parts that I'm using are spread across the keyboard I don't think I can just split the keyboard.

That is a GOOD assertion! A lot of people wouldn't realize that. That is why some drum kits have key assignments that are restricted to only a small portion of the keyboard. The rest of the keyboard is where the user plays the other parts.

What am I doing wrong?

Taking into consideration what I first said above you either need to split the keyboard range used for the different parts (drums vs. lead) or you need to modify the drum part to use specific velocity ranges that you, the human, can't really produce (such as a velocity of 1 or 126).

I was thinking I could maybe convert my pattern to an arp but I don't know how to do that and I didn't know if that would even work.

Good thinking - that is what many of the presets do. The example in that article uses several ARPs. The article also shows you how to find, and try, other preset arpeggios so you can hear what they sound like.

Creating your own arps is one way to go.

I know people generally don't like 'Read The Fine Manual' type comments but Yamaha has some excellent articles that show you how to do things.

This one by Blake takes you through ALL of the steps to create your own drum parts that use arpeggios and don't have the problem you are having.
https://www.yamahasynth.com/learn/music-production/blake-s-take-level-up-your-drums-3

MONTAGE and MODX feature over 10,000 Arpeggios. Almost half are drum Arpeggios. Right out of the box you have access to a huge library of sounds and drum grooves to inspire your creativity. I’ll show you how to check them out below. First, you’ll need to check out some drum kit. Press [CATEGORY SEARCH] and touch “Drum/Perc” in the Touch Screen:

Check out some of those thousands of drum arpeggios that are all set to go and work properly.

Turn off keyboard control for the piano Part while recording your drum sequence. The piano won't play and that MIDI channel (track) shouldn't record any events.

Turn off keyboard control for the piano Part while recording your drum sequence. The piano won't play and that MIDI channel (track) shouldn't record any events.

How does that prevent both the piano part and the drum part from responding to the same key press?

The piano keyboard won't send keypresses to the piano. The pattern sequencer won't record these for the piano channel so playback will only have the drum channel (Part/track) play back what you recorded.

Keyboard Control turns off the piano key presses and knob twirling for each Part that has Keyboard Control turned off (and not selected).

If you want to record one track at a time then turn keyboard control off for all Parts and then select the single Part you want to record. When you select a Part with keyboard control OFF - the knobs and piano keys twirled and pressed only get sent to the selected Part. Alternatively, you can turn off keyboard control for every Part you don't want to send keypresses to and select no Part. Now keypresses and knob twirling will be sent to all of the Part(s) with Keyboard Control on.

No matter what - whatever is recorded in the sequencer will be sent to the other Parts even if keyboard control is off. So you can record the drums with only keyboard control turned on the drums then turn keyboard control off of the drums and turn on keyboard control for just the piano and keep recording -- now only your piano will be documented and your loop will play the drums you recorded.

The key is use keyboard control to handle this. No need to create an arp.

The piano keyboard won't send keypresses to the piano. The pattern sequencer won't record these for the piano channel so playback will only have the drum channel (Part/track) play back what you recorded.

Got it! Thanks. I overlooked that the drum part was being played by the pattern and NOT by the user. The MIDI in the pattern recording will have the correct channel number for the drum part.

What I said would only apply if the user is playing both parts since there is no channel associated with a key press so the part/element info has to sort it out.

Wow! That worked perfectly. I turned off keyboard control for my piano, recorded my drum part and then turned keyboard control back on for the piano and it works like a charm! I can play the drum part and piano with no conflicts. Simple!

Thank you guys so much!

The piano will not respond to the keyboard's key presses. The pattern sequencer won't capture these for the x trench run piano channel, therefore when you play them again, just the drum channel (Part/track) will reproduce your patterns.