Though I also had some issues with the Helmet of Grit last playthrough, which also triggers once HP drops below 50% but sometimes did and sometimes didn’t. Though I never managed to work out what was going wrong there, either, so all I can do is feel your pain!
A bit more info about my Helmet of Grit issue in case it helps with the Moondrop Pendant one. I'd saved a game showing my character wearing the Helmet of Grit, <50% HP and 1 bonus action in order to submit a bug report to Larian. But I have just reloaded this save to check it before submitting my report, and when reloaded my character has 2 bonus actions! Fortunately, I had also taken a screenshot at the same time as making the save clearly showing a single bonus action or I'd start to think I was going bonkers.
But I see you've already tried to save/reload a game with the Moondrop Pendant not working, and it's not helped. So it does look as though the issues are probably unrelated, or at least present different symptoms.
