Were they just in the main iPhone photo library, also in shsred folders, just on the device or cloud synced as well?
Backups avail?
while the library is sorted in date order, maybe you have filtered something out so you could check your filters. Also check your hidden album/folders in case they were in advertently hidden in which case they would not show in the main library but a hidden album.
sometimes after software updates, I’ve personally experienced settings changing from the default condition. They once were to something different. For example, I’ve not cloud sync devices and then done a software update and suddenly all my photos are coming and going because cloud syncing was activated. Maybe your hidden folders are shown in case you couldn’t even find the hidden folder let alone and see what’s in it so you have to toggle the ability to see it and then check with him there as well. Unlikely but maybe.
in the Photos app, you can also search by typing in April as the search criteria or the date in April and see what is revealed. I don’t know that it reveals things in the hidden album/folder, but if you would’ve happen to been looking inside of an album instead of main library