Which MCS INIT have you chosen in UL and in DL ?
A higher MCS is weaker, and any interference or low coverage can lead to a call drop.
INIT MCS should be limited to 6 in DL, and in UL you can choose 6 or even a lower value.
There is no point from using higher MCS if it leads to TBF drops 😉 The MCS adaptation (from lower MCS to higher MCS) is a quite reactive mechanism, that will increase efficiently the MCS if the radio conditions are good enough.