Gallery app doesn't show downloaded PNG pictures

The gallery app doesn't show any PNG pictures I download. Downloaded JPG pictures are shown correctly just like PNG pictures which were created by/on the phone.

Example: When I take a screenshot it is saved as PNG and shown correctly by the gallery app. But when I download a PNG picture it is saved and shown in the files app but not in the gallery app.

So this is a test picture I created for this issue:

I sent it to my phone via e-mail. It was downloaded correctly to the "downloads" folder:

It is shown correctly in the file manager:

But where is it in the gallery app? It's not there! I restarted the gallery app several times but the picture kept missing.

This is my /e/ version:

phone: Fairphone 3 (FP3)