I looked at the edits made using the affected user rights manager gadget:

The first edit with this problem was on the 19.08.2023 (diff)

The last edit which automatically added the content as a new line was on the 16.08.2023 (diff)

The gadget was not changed during this time.

That would suggest the MediaWiki change (if there was one) happened in the 1.42.0-wmf.1 train (T348354). @ssastry do you think this could be related to the parser cache or parsoid changes you mentioned there?

So far, I haven’t been able to get the suspected previous behavior (that a \n was automatically inserted) reproduced on localhost, even when checking out older MediaWiki core commits.

appendtext and prependtext does not add newlines, there are to add directly to the end/begin of the existing text.

The userRightsManager gadget depends on a newline generated by the substituted templates (the appendtext is just {{subst: ... }} ~~~~), the template for the filemover right provides this empty newline at the begin, while the other templates does not, so all the filemover addition looks correct, while other looks broken.

Global replace is broken also in january 2023 - https://commons.wikimedia.org/w/index.php?diff=728886061 / Older https://commons.wikimedia.org/w/index.php?diff=614577825

Another gadget provides newlines since the begin in 2012 - https://commons.wikimedia.org/w/index.php?title=MediaWiki:ProcessFileMoverRequests.js&oldid=68687244

To add new section use action=edit with section=new&sectiontitle=...&text=...

I see, thanks for investigating!