There was an issue with parser function (T167238) related to the fact that function was never tested in the context of the whole project.
We need a test set that will actually check render result of all functions embedded in a wiki-text.
Maybe as browser tests, but try to avoid this if possible.
I bet there are some details that can be added to the ticket.
I'm gladly inviting you to specify corner cases and/or considerations in this ticket.
This can be done easily with the parser tests (https://www.mediawiki.org/wiki/Parser_tests) infrastructure we have. It should be pretty easy to create basic tests for each parser function / magic word, etc.