I believe it is a TEMS issue (or a TEMS choice). There is no link degradation during HO : the C/I and RXQual should be stable.
However the speech frames are lost during the HO execution. And -perhaps- during the HO, to reflect that loss, TEMS artificially degrades the measurements.
In other words : Voice Quality is degraded during HO, but not the link quality. Even though TEMS is saying otherwise…