Did you notice a significance increase of the EV range indicated by the car since you replaced the battery? In that case it's possible that the BMU was reset and it thinks that the battery has a higher capacity than it actually has. The battery gets depleted and disconnects because of low cell voltage.
The only way to fix it is to take it to your dealer and have them do a DBCAM to have the BMU relearn the battery capacity. See Richie's FAQ
question A.2 for a description. He gives a reference to the procedure in the workshop manual, hopefully it can help you point the local mechanic to the correct procedure.
In the mean time, remember to force the car in charge mode when the battery level gets too low to prevent this from happening to you.