I did not check on ios 10 but on iOS 12-13 this issue was not present and on iOS 14 it did appear. Is it an iOS bug or an expected behaviour? if it is expected how can it be fixed?

i have discovered that it depends on order of hold/unhold. If you need to swap calls you need first to put one on hold(CXSetHeldCallAction isOnHold=1) and then unhold another(CXSetHeldCallAction isOnHold=0). For me it did solve the issue.

what helped to me: Right-click on any opened tab, "Close other tabs". would not say it is now ideal, but much better than it was.