Cannot pull clear thumbnails for vimeo/youtube videos, cannot upload custom thumbs for external video.

Site URL: https://www.matikdesign.com/forumpost

scoured the whole internet searching for this problem and cannot crack it, when uploading videos directly to squarespace you can upload custom thumbnails, they look great! why cant we upload custom thumbs for external links (option is greyed out)

No matter what i have tried, uploading custom thumbs directly to squarespace/ vimeo etc, in a number of different resolutions it always comes out completely unusable, its very important for me as im sure it is for others, i would really rather not have to completely rebuild on another platform is there a fix/code etc for this??

Obviously i would just use the squarespace video's ideally but with the limited uploads its not sustainable.

i have set this test page up to illustrate the issue.

https://www.matikdesign.com/forumpost

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Started experiencing the same problem. Any fix so far?