Thanks heaps Seth for reporting.
This certainly sounds very frustrating, I just tested on Linux+Firefox and am not getting a repro.
Created a DM to myself, CTRL-SHIFT-ENTER and it posted, then replied and posted.
I am wondering if we have a Mac quirk here. What browser are you using? (Safari?)
I think our first port of call here is confirming someone on Mac is able to reproduce this.
We will get this sorted, for sure!