Been having this issue sporadically for about two weeks now. These are identical parts. I tried with and without Studio Build Suite (my preferred building plugin) since that has a section tool that’s different from the default. I found that selecting the Part in Explorer allowed me to finally delete the part in the video, but I can’t confirm if that’s a consistent workaround yet. EDIT: can confirm this is not a consistent workaround 
Also the delete/copy/etc remains off even after deleting another object, so @veldvin I can confirm that it is not a 100% workaround. It might be a workaround sometimes or was fixed on your end from some other unrelated action.
Also @nowodev I just want to set the record straight that Roblox concurrent player counts would have zero affect on Studio functionality, especially considering I’ve had this problem before this high concurrent player count, and haven’t have this problem when Roblox has had high concurrent player counts in the past 
Just wanted to say this because I always see a lot of people blaming player counts or any variety of Roblox issues when that’s never the issue. Thanks for the correction, I see you were referring to API issues