Incomplete data when querying with Flux


currently we’ve got a problem querying data from InfluxDB using Flux. We’re currently using version 1.8.3-alpine, and try to display the data in Grafana v7.5.8 (0184ed92b5).

The problem we have, is that we sometimes do not get all the data from a given time range. This started in July and has recently gotten better, but still we’re not sure if it might come back one day. Also, when looking at data from July, the problem still exists.
The problem seems to occur, when the time range queried contains Sundays on midnight (00:00:00 UTC). E.g. when querying from Thursday 00:00:00 UTC to Wednesday 23:59:59 UTC, I only get the data from Thursday 00:00:00 UTC to Sunday 23:59:59 UTC. If I want the data from Monday through Wednesday, I need to start the query on Monday 00:00:00 UTC. So usually, you can only reliably query data from Monday 00:00:00 UTC to Sunday 23:59:59, which is quite annoying, if you want to see the data of the last 7 nights at once.
It does not seem to be related to the data in the DB. The last entries are usually written between 23:55 and 23:57, and if I query from Sunday 23:59:59 to Wednesday 23:59:59, I get no data at all.

And the problem is even weirder: Some of the weeks are “compatible” with each other. When querying from Thursday, 2021/07/15 - today, I get the data from Thursday 2021/07/15 to Sunday 2021/07/18 23:59:59 UTC. But when querying from e.g. Wednesday 2021/07/07 - today, I get all the data from the following days:
Wed 2021/07/07 - Sun 2021/07/11
Mon 2021/07/19 - Sun 2021/07/25
Mon 2021/08/02 - today

When using InfluxQL for the same time range, or using SQL outside of Grafana, this problem is non-existent.

Has anyone a clue what could be the cause of this, or if this is a known issue that is fixed with a more recent release? Thanks in advance.