I have seen this pattern before. MT UE gets PS paging but does not receive it due to bad SINR in DL. Later no more PS paging but CS paging for CSFB. So your cause of issue is bad RF on MT UE.
Can you check and share the volte capability of UE and volte is enable or not for called ue?
for UE capability you can check the featureGroupIndicators bit 3 and bit 7. if these 2 bits are 1, it means ue supports volte.
for checking the activity of volte, you can check the Voice domain preference for E-UTRAN: IMS PS voice preferred, CS Voice as secondary (3) option from attach/ ue capability message. some times these messages are sent in handover required message as well.