Update: Issue resolved, the issue is related to GIF size and sometimes because of GIF Width and Height (most probably, prefer a GIF of landscape type)
The cover pic was a GIF
imgproxy there is some issue, because I can view the image in the draft and the preview screen. This tell me that there is no problems with uploading and cropping, so it should be related to fetching.
Leeawrdslope is a Forem based community, but not using the traditional self-hosting guide. It is based on dokku and doesn't use imgproxy; for image caching and resizing we are using Cloudinaries Immaga Plugin.
Here is a post on Leewardslope, which holds a GIF as cover picture.