1) Is any sustain pedal involved?

2) You have a MIDI routing issue within Logic.

I am not a Logic Pro User myself but perhaps you'll find some help here

1) No. I connected Montage to a powered USB hub, and it might have resolved it - but have to test more to make sure. I'm using Montage Port 1, by the way.

2) No, the reason was that in my the Performance, I had the "Keyboard Control" switches on. If recording MIDI on a DAW, do they need to be off? Where can I disable them globally so that I don't have to do it each time I load a new Part?

Hub? Hubs are not recommended for exactly that reason.

No, the reason was that in my the Performance, I had the "Keyboard Control" switches on. If recording MIDI on a DAW, do they need to be off? Where can I disable them globally so that I don't have to do it each time I load a new Part?

I'm not sure of your workflow, but typically when working with a pro DAW you are working with your controller set to LOCAL CONTROL = OFF, allowing the software to channelize and route the data.

Without knowing what you’re attempting, it's impossible to say what you should be doing.
Using one of the templates "MIDI Rec on DAW" without knowing if it serves your needs could be a flaw in your workflow.

I love Templates, but I only recommend them when they specificially address the needs you have... that QUICK SETUP makes eight, count them, eight settings concerning recording the MONTAGE to a DAW. It is a user customizable template.

Fail to review and learn those eight parameters at your own peril. You are correct, one of the eight settings it memorized is that LOCAL CONTROL is OFF, this stops the MONTAGE keys from triggering the MONTAGE tone engine directly. You then must setup your DAW to (intelligently) route the signal from that point.

KBD CTRL is a MONTAGE setting that allows you to link any or all of the first eight MONTAGE Performance PARTs into one playable entity, occupying from 1 to 8 MIDI Channels, Transmit/Receive.

If you understand how your DAW echoes (thru's) the data from an active Track you can control where it goes. Some DAWs simply Record all incoming MIDI data as is and send it back just as it came in, some DAWs Record incoming MIDI data and rechannelize it before it echoes it back to the tone generator. (What does Logic Pro do?)

If you are transmitting in on multiple MIDI channels from the MONTAGE make sure each channel is discreetly routed back to its source.

A big flangey swirling sound usually means you are recording the same MIDI data multiple times or you are sending all the data you recorded back on a single MIDI CH.... because your DAW is one that rechannelizes all data to the same channel. If that is the case, find an option that lets the data go back out on the original channel it arrived on.

If you can set the MIDI IN channel on a TRACK make sure it is only recording one specific MIDI channel. You may need to filter all but that single MIDI channel.

Hub? Hubs are not recommended for exactly that reason.

I'm aware, and that is why I connected it straight in at first - but recall some folks having luck with hardware instruments by connecting to a powered hub, and thought I'd give it a go.

If you understand how your DAW echoes (thru's) the data from an active Track you can control where it goes. Some DAWs simply Record all incoming MIDI data as is and send it back just as it came in, some DAWs Record incoming MIDI data and rechannelize it before it echoes it back to the tone generator. (What does Logic Pro do?)

If you are transmitting in on multiple MIDI channels from the MONTAGE make sure each channel is discreetly routed back to its source.

A big flangey swirling sound usually means you are recording the same MIDI data multiple times or you are sending all the data you recorded back on a single MIDI CH.... because your DAW is one that rechannelizes all data to the same channel. If that is the case, find an option that lets the data go back out on the original channel it arrived on.

I think you're right - it's most likely this MIDI Thru / echo issue you described. I'll need to investigate.

Thank you for your quick replies! I'll get back to this.

I believe previous success with hubs was limited to the "To Device" port on Montage - not the "To Host" USB port which connects to a computer. Even this "To Device" usage was in unsupported configurations.

The "To Host" is always point-to-point and always to a low current target (Montage) which itself is always self-powered. There should not be any reason to add a hub between the "To Host" downstream target (Montage) and the upstream host (PC).

Article: MONTAGE MIDI Settings Explained