There is what appears to be an errant hardcoded semicolon (";") appearing between separators of the watchlist.
Originally reported on enwiki in: https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)#What%27s_with_the_semicolons_in_the_watchlist%3F
Seems to be related to the coding for:
::after
which is loading:
.mw-title::after { content: ';\00a0'; }
Screenshot: