Maybe you need to turn MODX's direct monitor setting off (under [UTILITY] ).

I'm a long time DAW...Synth...MIDI...Audio Interface guy...just recently purchased a MODX for my home studio....fantastic playing synth...straight out of the box I went through the sounds and drum patterns w/ headphones...awesome!...but when I started to play through my Audio Interface (UAD Apollo Twin)....everything sounded fine with two lines in for stereo output...except for the Drum sounds...I'm getting a weird flanging effect coming back on just those sounds....w/ headphones not at all....even when I'm setting up in my DAW...flanging on input and output...What could be causing this?

This occurs when you monitoring two pathways for the sound (audio) to reach your speakers.
It is normal to have to select which one you wish to monitor. You should never monitor both or you will hear the “direct” signal, and the “latent” signal.

The “direct” signal is the one that takes the shortest route from the source to the speakers,
The “latent” signal is the one that gets time stamped, processed and route to the speakers.

The direct path is speed of light, the latent signal is delayed by the few milliseconds your computer takes to receive-process-and thru your audio input (the DAW will typically measure and report the amount of latency.

If you are using an audio interface it will typically have a method to “Direct Monitor”, and certainly your DAW has a way to mute monitoring what you are recording (the mute is post the signal being recorded, so you can choose to mute the Audio Track, until you’re ready to playback).

The reason you are not hearing it in headphones connected to the keyboard because it is not a problem beyond your selection of which to monitor. The headphones connected to the keyboard includes only the source. Headphones connected to your audio interface will include the data coming from the synth, as an input going TO the computer, and it will also have audio coming FROM the computer as the recorded signal.

In your Audio Interface, make the selection to use either the Direct Monitor or not. If you turn Direct Monitor Off, then you will hear only the signal AFTER it is documented in your DAW. it will still be a very few milliseconds late, but it will be alone (no flanging).

Why you are hearing it on just the drums could be that it is most apparent on the drums. We’d need to have a bit more information on your setup.

Extra Credit:
The term “flanging” is a term that has its origin in the recording studio back when reel-to-reel tape ruled the world. Raw Magnetic Tape came tightly spun around a hub... you had to metal devices called flanges, that you placed under and over the tape before you placed it on the machine. To create the “effect” called Flanging, you would Mixdown the signal you wanted to effect and feed it to two 2-Track machines. They both would record the same data... and sound in ‘perfect’ synchronization... to create the effect you would momentarily place a thumb on one of the flanges - this created a very slight delay - the two identical signals were now a few milliseconds apart. Voila! Flanging!
Originally it was a very expensive effect to recreate... it required two 2-Track machines and some routing skills. We used to recreate this with to Scully or Revox machines (so it wasn’t inexpensive to do)... some where in the late 70’s Eventide made two space rack module ”Flanger” (and that’s all it did)...