Search is made possible by creating a local index of your events and storing it on your device. When you search for an event, the app goes through the index of your downloaded events and highlights all matches.
Awesome, it took a while due to the nature of end-to-end encryption. Now my subscription for Proton Unlimited is a bit more worthy. :)
Well, I uninstalled the app and installed it again. Now the upload does work properly.
Thank you for the “black magic”.
Edited: Well, I tried to upload some more images—more than 100 images. And it didn’t work again! Same annoying problem!