I think maybe after the call finished, the Re-Selection priority back the call to the 3G ZTE site before 4G.
Did you check 3G-3G reselection parameters?
You can try with remove relation in the way from NOKIA - ZTE.
And try again.
It can be because of higher reselection priority on ZTE freq.
After call release, the highest priority RAT is selected as per strategy.
LTE being the highest should go.
So you need to check the CRP.
1st case call start at 3G ZTE.
2nd case start at 3G NOKIA.
I think you should start to check how 3G-4G performed at 2nd case.
through which triggering condition.
Then you can analyse why it doesn’t work at 1st case.
When the call ring (begin) in ZTE the UE are in idle or doing a data session?
For your explanation this happens sometimes when the UE don’t release all services (E-RAB)…