Missing data, amount of missing data depending on selection time range


I am evaluating whether or not Grafana can help monitoring machinery.
We are using Grafana v7.5.5 (b5190ee547) on Ubuntu, using a MariaDB database.

(I know that MariaDB is not supported directly. We the MariaDB via MySQL data source. The basics are obviously working. If we decide for Grafana, we might have to switch to PostgreSQL)

The problem can be seen in the following screenshots.
The only thing that is changed from one picture to the next is the ‘Relative time range’, going from 24h, to 12h, to 6h and 1h. (edit: I am only allowed to upload to pictures per post. The other pictures will follow in a new post …)

A part of the data is not displayed. Obviously the data itself is present.
Especially problematic:
Most recent data is not displayed at all.

Any ideas what’s going wrong here / how to fix this?

Thanks and regards,

The other screenshots: