The May/June 2023 firmware updates from Oneplus and Oppo to ColorOS 13 seem to reset the Android calendar storage (this is where all calendar data is stored in Android).

For local calendars this means full data loss, unless you have a backup (which is always mandatory for important local data, as otherwise device loss / failure / sw or fw-failure will lead to data loss).

For cloud calendars this should trigger a full-resync (which may take a little, but should recover all data - except if there were unsynced local changes).

aCalendar has no own calendar data. It is a user interface to the Android calendar storage. We have no access to any of your data and we have no way to recover lost data from a broken firmware update. Please contact your phone manufacturer for more details.