Testing right now...
It seems that save bug issue can be reproduced without any AI inits in your vicinity and in single mission too...
Please tell me can you confirm this:
Dive to something like 170m with 2/3 ahead speed and using TC no higher than 16. This should not collapse game saving function...
Then, gradually increase TC to 32/64/128 (regardless of 3D/Nav map view) and you'll probably notice that at certain moment engine speed gauge will suddenly drop to "all stop"?!

This is the exact moment when save bug kicks in...
What do you say?