Triggers run in "system mode", where user permissions and sharing are not applied, so it doesn't matter if the code is with/without/inherited - there simply won't be any sharing constraints.

Triggers run in "system mode", where user permissions and sharing are not applied.

By delegating to Apex "with sharing" you can get the current user's sharing to be applied.

I am guessing that, because triggers are a bit "special", there may be some different handling of undeclared (and perhaps explicitly declared) inherited sharing, even when invoked indirectly. I don't know if it is a bug or simply undocumented behaviour.

There is some possibly a little out-of-date detail in this other thread that may be of interest.