I've used Proton for like 5-6 years now, Calendar is definitely the roughest of their products. Good news there is they are rebuilding it from scratch with an initial (beta?) release scheduled for this year!
> To support much-requested features like tasks, search capabilities, and offline access, we’ve started work on our next generation of Proton Calendar apps for iOS and Android, which we aim to release toward the end of 2025.