In en.Wikipedia, I log in and go to my watchlist, which defaults to 1,000 items and 30 days, but it retrieves only a few days, sometimes with about 50 items.

It turns out that the solution is to exclude Category. I don't even have to exclude Category Talk. Instead of excluding, I can explicitly include everything except Category. Thus filtered, I get late April to May 18. With Category, I get May 14-18 only. The time stamp of the earliest item on May 14 varies. I don't know how to get 30 days of Category namespace items.

While Category alone truncates the date range, Category with one or more other namespaces truncates too, but a little differently.

See also, all in Wikipedia's Village Pump (Technical), in reverse chronological order:

• Watchlist Too Short Again
• Did Something Break Watchlist and Make it Too Short Again?
• Watchlist or Recent Changes Showing Too Few Changes, Kludge to URL

(Apparently, there's no Phabricator tag for anything like en-WP.)