Edited/Deleted recurrence instances are sometimes reverted to previous state

[sunkup]

Problem scope

• [ ] I'm sure that this is a DAVx⁵ problem.

App version

• [X] I'm using the latest available DAVx⁵ version.

Android version and device/firmware type

No response

Steps to reproduce

Install radicale, create a calendar and add it in Thunderbird and DAVx5, then:

1. Add a recurring event in in thunderbird
2. Sync in DAVx5 and verify it shows up in the calendar
3. Delete one of the recurring event entries in the calendar app (probably not the first (main) entry)
4. Sync manually in DAVx5 or wait untill Sync Adapter Framework initiates sync

Actual result

Sometimes (often after fresh emulator boot, or radicale startup?) the event gets added back in the calendar and is not deleted from Thunderbird.

The same happens when editing an event instead of deleting it. The changes are reverted then.

Expected result

The changes should be synced, not reverted.

Further info

Captured debug information when it happened (developers only): https://cloud.bitfire.at/f/567372

There are 412 precondition failed error response codes in the logs.

Discussed in https://github.com/bitfireAT/davx5-ose/discussions/818