Not what normally happens
I am sure that it is possible to do the direct line to UPG in an Otter, but it's just not how it normally happens.
Most common routing would be out to the west of those mountains where the navaids are :- Rantepao, Makale, Enrekang
That track adds a few km and about 5-10 mins to flight time.
7000 feet is entirely consistent with common assigned altitude for these flights.
My guess (yes GUESS) is that he was 20 - 30 km east of favoured track for some unknown reason, in cloud or with limited visibility.
Lots of smoke around just now but shouldn't have been a problem at that altitude.
It'll all come out eventually.
Does PK-KBD ring a bell Peekay4?