When I'm adding multiples tags, it seems only somes are used, without any notice or error message.
Am I missing an important thing or a missing patchnote ?
Check out the help pages on search. You can reach them from the advanced search tips accordion on the search page.
To combine results from multiple tags, separate the tag names (enclosed in square brackets) with the word "or":
[widgets] or [geegaws]returns questions tagged with either tag.