It's happening to me now, on my Android tablet and Vivaldi browser. No idea why, but logging out and clearing cache don't fix it.
No idea if it's related but I noticed that the message I type in the reply box (without the reply button!) is in a different font, so something is happening to the properties (or somesuch) of the reply box.
I've switched to my laptop to do this post, I'll reboot my tablet and see if that resolves it.
Edit: I maybe had a reply saved/cached/whatever in the reply window already, not sure. I don't know if that would lead to something funky happening to the reply function.