We've faced with issue that part of elements on our qml based application disappear.
In some cases it's 100% reproducible and in other not.
Also we've found workaround to fix this issue. Apply "opacity: 0.9999" to element that disappear. But it's working only in place where it's using. I.e. if you apply opacity inside qml component it's won't work. So you need apply this workaround everywhere were issue appears. Because of that we cannot use it.
Furthermore we cannot reproduce this issue at Qt5.15. Maybe it could help.