Following text please take with reserve, this is just thoughts of mine:

I’m not sure which part of motor chamber is on this picture, but let’s presume it is rear side, side in which boost part of propellant is placed. Why, because of this longitudinal tearing.
When I wrote how to take out propellant from motor, heating and extraction is one way, the other way which is much more easier and faster, just take the motor, place it on stand and fire it. I believe Almaz did it this way, why to bother with complications.
Now let’s see one sample as preamble, rocket motor of 9M33 OSA

It has two grains, boost and sustain. Boost is one cylinder burning from inside and outside while front and back surfaces are inhibited. Sustain grain is cylinder with 4 slots on rear side, it burns from inside channel, rear side and through these slots.

We can see these burnt areas, this is area of these slots. This is place where chamber was exposed to the flame from the very first beginning, just like rear side where boost grain was placed, only here because of geometry, additionally these areas were exposed to erosion.

It can be seen how thermal insulation is longitudinally damaged, removed, grinded, and chamber made of steel got exposed to the flame and temperature. Let’s not to forget, we are talking about temperature of some 3100 degC.
This view is even better ->

Rear part of the chamber together with nozzle was cut and removed, we can see these 4 areas, 4 stripes. On the bottom we can nicely see thicker layer of insulation, this is where sustain grain was placed, this is most likely what remained of it, inhibitor placed on external surface of sustain grain.
Now back to the BUK…

Boost side of the grain before start and after 3-3,5 seconds…of course flame will get to the chamber first where slot spikes are, this is area of pronounced erosion, this is area which will be exposed to flame and heat the longest. What we can expect is that some signs will appear following these 7 areas, also undoubtedly these areas are because of heat weakest points, steel exposed to such temperature simply lose its strength and common sense would say that if chamber will tear, it will tear there.
To repeat, take all this with reserve, I’m not professional expert for these stuffs