For example:

• https://lichess.org/games/search?perf=5&dateMin=2023-07-01&dateMax=2023-07-31&sort.field=d&sort.order=desc#results
  Standard: 0 games found

• https://lichess.org/games/search?perf=2&dateMin=2023-07-01&dateMax=2023-07-31&sort.field=d&sort.order=desc#results
  Blitz: 10,000 games found

Same happens with the advanced search on a user's profile.

Maybe I'm missing something but I've tried it with different dates, filters, etc. and on Windows & Android, Chrome & Firefox.

Games are found as long as the "Standard" variant filter is not selected.