The case conversion function seems to have an error in the latest mp3tag (v3.37) version. It converts P into p. Up to now this is the only character I have noticed.
The odd thing is that it doesn't do that on all words but e.g. "People"" is converted into "people".
Please show us a screenshot of the action parameters that you use and a field in which this occurs.
How do you call that action?
Could it be that you have another action activated that treats just the word "People"?
Please check the Actions dialogue how many actions you have selected.
Can you try replacing the space character before people with one you've entered yourself and try again?
I've just checked the release on Bandcamp: it's U+00A0, a non-breaking space which differs from a normal space character and is usually not found in track titles.