This issue turned out to not be an gcc issue, but a badly declared flexible / 'zero-length array' at the end of the structure, which then relied on undefined behaviour. The declared size (here [4]) was then apparently taken into account in the code generation.
I do not know of a way to diagnose this kind of use without warning for basically all kinds of arrays at the end of structures. Though it would be nice, as it was not straightforward to find the issue.
Perhaps a note in the gcc-14 upgrade notes that the compiler now uses declared array sizes (more) to influence loop execution might be useful.
Except for that, this bug can be closed.
