FWIW, on my Wikipedia screen, if put the cursor on the bell icon's clapper, the tiny bump at the bottom, I get Alerts; but if I put the cursor anywhere else on the bell icon I get Notices.

Thanks for tracking down the edit that caused this problem. I contacted the editor who made it and he reversed it, and so the problem is fixed. Apparently the same text string is used in the “List of abbreviations” box in the upper right of the Watchlist and the markup does work there. The fact that the string is used in two places with possibly different results should at least be documented somewhere.