GarageBand random sustain on notes

I recently bought a SubZero midi keyboard to plug into my Mac and use with GarageBand. Once I have been playing for a little while, it always happens that one or two of the notes will sustain after the key is pressed. This problem will go away for a short time if I switch to a different software instrument, but then will happen again on that instrument (with a different note; I haven't detected any pattern there).

I have tested the keyboard on other software and with a simple tool to test that the keys are turning the input on and off correctly, so it seems to be an issue with GarageBand (and maybe how it interfaces with this particular keyboard?). Any ideas for how to fix things would be appreciated.

I am running GarageBand on a 2019 21.5inch iMac with Big Sur 11.2.1. The keyboard is a SubZero ControlKey49S. I also tried on my older MacBook Air and managed to replicate the problem there with an older version of GarageBand.