The Google “Calendar” app is included in every stock distribution of Android, including FPOS running on the FP4. However, as I understand it, you are using the Proton Calendar app, in conjunction with a proton.me account, is that correct?
You may be experiencing a problem introduced when Fairphone made changes to touchscreen responsivity in an update this summer. For more information please see here (it’s a very long topic):
I think you would be well advised to contact official Fairphone support (for more info, click contactsupport )