There seems to be some kind of bug where gcovr 5.1 won't parse branch data with negative counts. [0][1]
It's not entirely clear (to me) whether this is a gcovr or gcov bug [2], so for the time being downgrading seems to be the easiest option.
[0] https://github.com/gcovr/gcovr/issues/610 [1] https://github.com/gcovr/gcovr/issues/583 [2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67937#c6