A few weeks and many tests later and the problem persist.
Definitely, nothing to do with the built-in TTS feature. The problem seems to appear randomly in all kind of cards, and doesn’t affect only to the space bar, the keyboard just doesn’t respond from time to time and then, when I hit the key again, it has a quick double response. For example, if I press the space bar while reviewing a card and the program doesn’t respond, and then I press the space bar again, goes to the next question very quickly, as if I had pressed the space bar twice very quickly.
It’s not a keyboards problem, I’m experiencing this problem only in Anki, no other software is affected. No addons used.
Not a serious problem, just a little bit annoying.
I forgot this: just for reference for future readers, I haven’t had this problem for months now: not 100% sure, but probaby since version 2.1.48 the keyboard is responding smoothly. I didn’t do anything special, at some point the problem simply was gone.