Then the only thing we can say at this point, with the information provided, is that you have undefined behavior
somewhere in the program. It might be somewhere unrelated to the vector itself. The only way to find out for sure is to use tools like Valgrind, compiler sanitizers, static analyzers, checking for out-of-bounds problems, and code reviews. If the tools doesn't help, then book a meeting with all involved developers, and the management needed, and do a codereview as a workshop. —
Some programmer dude 38 secs ago