Phew, finally.

After git bisecting, I have narrowed down this bug to this commit: http://git.savannah.gnu.org/cgit/emacs.git/commit/?id=0644e6f56d2be82dd716478eb65e7b3c761d813d

So I ended git bisecting at this point:

km²~/downloads/:git/emacs> git stash save && git bisect good

Saved working directory and index state WIP on (no branch): 1f55925 ; Fix breakage from previous commit

HEAD is now at 1f55925 ; Fix breakage from previous commit

Bisecting: 0 revisions left to test after this (roughly 1 step)

[0644e6f56d2be82dd716478eb65e7b3c761d813d] Fix copying properties in 'format' when it produces padding

Building emacs using that commit shows that bug.

I don't understand what the C code does. But the comments make somewhat sense as that code change has to do with the format function, and thus related to the incorrect text properties of eldoc hints.