Then I don't know how it happens, because I only have yellowish background + black and red defined in my terminal (also white, which I don't see used anywhere). And here is a screenshot from my Terminal.app, where the lifetime is gray: <https://0x0.st/HqVq.32.png>.
I've checked and the 10th colour is set to #000000 in my Terminal.app settings.
If SGR 38;5;10 is not producing green, which it isn't, then it must be following your palette. My educated guess is that for some reason it believes that it should lighten the palette-specified colours from 8 to 15, and it's lightening your black to a very dark grey.
I've checked and the 10th colour is set to #000000 in my Terminal.app settings.