I have a list with about 250 rows and a column called "Photo" that has relatively low-res .jpg images.
It works pretty well in list mode, but then I switch to the Gallery view everything breaks apart. Only rows that DON'T have images have their cards show up. Like this:
I even tried creating a new view and have it be gallery by default. Same result:
I should point out that the pictures were added to the list via
Set-PnPListItem PnP.Powershell module cmdlet. I don't know if that could have any impact on this or if it's just SharePoint not being able to handle 250 thumbnails in a gallery. It works perfect in List and Compact List formats.