When using to_date with a format mask I think most people assume that the format mask is enforced ... which it isnt (see attached screenshot).
Also leaving out numbers wont always result in an exception.

So my suggestion would be to always use ‚FX‘ in front of a format mask to enforce the mask except you need it to be ‚fuzzy‘.
See docs:
https://docs.oracle.com/en/database/oracle/oracle-database/21/sqlrf/Format-Models.html#GUID-7FF68D9E-C7E2-4CA1-9DDB-5CC7169EEEEA