The F-Droid application detects active VPN connections as mobile data when connected to wifi.
This causes the issue that the application-pictures are not loading because the default setting of the slider for mobile is not to load pictures (slider in the middle).
Most simple way to try this out is to install orbot and run it as VPN. Then there is no need to get some VPN account.
I have not found this issue anywhere else. Is this issue known?