None of the following appears to be documented, but based on the experience collected from posts already listed in the OP.
More solid information might be posted.
Is this behavior expected?
No. This is quite different from what many people would expect.
If not, any idea on what could the problem be?
It appears that this behavior is standard in Outlook (i.e., not something that I see in my system due to configuration issues). So, the "problem" is how Outlook is designed.
... I meant to... find a solution to revert this behavior (if possible)...
There appears to be none.
... instead of workarounds.
As mentioned here and in the OP.
PS: Since none of the two workarounds proposed appear to be options in some cases, one may try to write a macro that would execute a shell command with touch ...
(or the like), to set the modification time. It will likely find that VbaProject.OTM is in use, so one would have to deal with that too. Not a trivial task.