So I am guessing but it’s an educated guess.
What I suspect is happening is your connection swap from headphone to headset when the call is made so your headphones then connect in headset profile at BT 5.2. It is then stuck at that and cannot connect back to 4.2, so you have to do exactly what you said of off/on.
I can’t think of a clever workaround for this, it relies on a firmware update to fix it. I’d just ensure email to support so it’s added to their to-do list.
Ironically an older bud on BT 4.2 would not suffer this problem.
I suggest the issue is Soundcore is not testing adequately, it’s like they only test using two BT 5 products not trying different sequences of 4.2 5 5.2 so it’s not detected until you’ve bought it and reported it.
If you didn’t want to wait I suspect if you bought a BT 5.2 bluetooth dongle for your laptop so it keeps on 5.2 the whole time it would then work seamlessly.
BTW what I’m doing is keeping my buds on the oldest version - I’m still using old buds which work as then they always connect at the same version - an old one. I can’t make Soundcore test products properly but I can avoid the causes of their inadequate testing. I also offer myself to do testing, I’d certainly be on this exact issue more than those who connected to their phone and said “sounds great”.