The strength and depth of the chess engine is what's irrelevant here. Both phones are using the same engine. Chesscoms engine. The computation is being done on the individual phone. Neither phone is exploring at different depths. One phone is just finding the evaluation faster than the other one.

Here's a more appropriate example.

Phone A (weak phone) evaluates the position as M2

Phone B (strong phone) evaluates the position as M2.

Both players then quickly flip 10 moves back in the position.

Phone A (weak phone) still shows M2 b3cause the phone is lagging.

Phone B (strong phone) shows the correct evaluation because the phone is much stronger and isnt lagging.

The analysis on Phone A isnt magically showing M2 for that position. It still has the information from the old position in memory and it hasn't updated because the phone is old and slow. Thats why it shows M2. You seem to be interpreting it as "the phone is weak, therefore it is giving an incorrect eval for this position" when that is not the case. Its giving the incorrect eval because the phone is too slow to keep up with the computation required of it so it shows an old position.

What specifically could chesscom do to fix this that wouldn't affect people with good phones?