From this, I wonder if the pendant might only work once per long rest, as opposed to every time your HP drops? I don’t think it says that on the description, but just a guess from your outline of the problem.
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!
