When I receive a Whatsapp call with my S20 FE clear view case closed, the ringtone rings 2-3 sec and stops while the call is still in progress. Very anoying. I miss calls!
This bug seems to have been listed for several years!
No fix yet Samsung ?!!!
I have a similar issue with the Samsung clear view case on an S20+. As you, when closed (and the phone is locked) I suffer delays what appears to be disconnections. I tend to use WhatsApp Web on my PC but this is not the issue, it just highlights the fact that I can see what is happening without opening the phone cover.
I only just purchased this phone and cover and my previous Samsung phones (S9+ and Note9) which had LED covers, had no issues at all.
There is a serious compatibility issue with the Samsung clear view covers and WhatsApp and possibly many other apps I don't use.
I decided to buy the LED cover for the S20+, and while it is cheaper and not as nice as the Note9 version, nor as nice as the clear view cover for the S20+, it appears to have fixed the WhatsApp issues I was having. The LED cover may also fix the issue detailed in the OP. MORENOSABATER doesn't say whether he has tested this with cover on and cover removed from the phone, but I assume he has.
From my findings, I can say there is a compatibility issue with the S20+ (and maybe S20FE), the clear view cover and WhatsApp. Perhaps a future firmware release will fix the issue, but now I will stick with the LED cover.