Add Release (single or VA)
Enter basic release info (anything)
Go to tracklist tab (either basic or advanced), but you must have previously had "Track names start with a number" unselected from an earlier editing session.
Click Add Disc
Click Existing Tracklist
Search for something, e.g. Pretty hate Machine by Nine Inch Nails.
Select it, click Add Disc

>>> Problem 1: Titles now have track numbers in them
>>> Problem 2: Click straight to "Edit Note" - Recording associations are now all broken (possibly because of track title changes); you'll be asked to confirm.
>>> Problem 3: Click back to Tracklist tab. Now your titles are like "1. 1. Head Like a Hole". Every time you go back and forth from tracklist and recordings tab you'll get added track numbers prepended to the titles.

This appears to be the same behaviour in basic and advanced tracklist editors.

Adding an existing tracklist should not be sensitive to a user setting like this; even less so if you have the Advanced tracklist open, where this setting doesn't make sense.