Using Android V10.3.4325 or iOS V10.3.4422 and a eg: Gmail calendar on OS15 & iOS26
When you first open the app and go to the Calendar section, you then press the Menu to see what calendars are selected. It has a check on Calendar and Holidays “but is highlighted on Holiday” and not Calendar as I left it.
So I press on Calendar to deselect it and then press on Calendar again to reselect it and its now highlighted on Calendar.
However if I then go back to the Mail section and “close the app” and then “reopen the app” and go back to the Calendar section and press the menu, “the Calendar highlight has gone back to Holidays”
So there is some bug in the app that is not remembering what calendar is highlighted. This has also been an issue with previous versions.