encodings.search_function is triggering this assertion at Python/ceval.c#L4055:

       TARGET(JUMP_ABSOLUTE) {
            PREDICTED(JUMP_ABSOLUTE);
            assert(oparg < INSTR_OFFSET());    <<<

It seems to be saying that absolute jumps should only go backwards?

The assertion is triggering reliably in a Windows debug build (including as part of the build process, which means we can't release right now). I don't see anything Windows-specific here though, so I assume it's core and may just be a codepath in encodings that Linux doesn't use?