While trying to debug an issue with the timing of tasks I was importing, I realized the error only appeared for recurrent tasks.
Please see the attached tasks, they differs very little:
diff non-recurrent.ical recurrent.ical
< SUMMARY:Not Every January 29
> SUMMARY:Every January 29
Only the UID (to avoid collision), the summary (to be readable) and the RRULE (present/not present) differ.
All other parameters (in particular DTSTART & DUE) are equal.

However, after importing it:
  • The non recurrent task has: "Start date 29/1/2021, 00:00" and "Date completed 1/2/2021, 23:00"
  • The recurrent task has "Start date 30/1/2021, 00:00", "Date completed 1/2/2021, 23:00" and the correct recurrence "This task recurs Every 12 months on day 29."
As you can see, the recurrent task is one full day after the non recurrent!
This is using frontend 7.10.4 Rev19 and backend 7.10.4-Rev18 (last patch releases for 7.10.4)
Reproduced on both Firefox 85.0.2 and Chrome 88.0.4324.182.

Screenshots attached.
Let me know if you need any more information/if I can help.
Attached Files