Monday-only bugs and similar are not unlikely with code dealing with weekday ordinals, when moving between locales with different conventions (is Monday or Sunday the first day of the week?). Or can cause any number of confusions if different bits of code make different assumptions.
I'm not sure why that would affect something like webpack though (unless the problem was specific to a date manipulation library).
Edit: spotted the post that arrived as I typed the above. Looks like an advertising related function that only ran once per week using something not available on all sorted OS builds for that code, so not a locale related problem. Advertising causing unexpected problems yet again!