A bunch of us use Multiplicity to multi-box across different computers in EverQuest.
It's super reproducible in this circumstance and causes us all sorts of trouble (push to talk not being released when I release a button when transitioning between computers while speaking in Discord might be the easiest repro step for you).
Install Discord (discordapp.com) on your main computer that is the server.
Set up a Push-To-Talk key in Discord and hold it down. Ensure that you see in Discord that you are transmitting as a result of your PTT key being held down.
Move the mouse to a second PC via multiplicity. Release your push to talk key. Notice that you are still transmitting in Discord.
I believe the issue is that when multiplicty transfers seamless control between 2 PCs, it doesn't signal the end of a keypress to the current machine that is receiving it.