Weird Delta-IV behavior after damage.
 

I was playing the bastion single mission, and saw a Russian captain use some really unorthodox tactics after taking damage.

I hit the Delta IV with a Mk.48. It immediately attempted an EMBTB, but ran out of steam before it reached the surface. The boomer must have suffered a massive flooding casualty, because it went straight to the bottom. But that wasn't the end of it. Somehow the sub survived hitting the seafloor at 24 knots and then proceeded to skate across the bottom as if it were an ice rink, maintaining 24 knots. I waited a good 5 minutes before finishing it off with two more mk.48s, but the boomer kept skating right until the end. It even attempted to evade my torpedoes.

I can beat that -- I had one do an emergency blow and penetrate an iceberg where it remained (untouchable)

The AI needs a little work I think - The Rigas especially have a habit of beaching themselves (My guess is the AI is working as it should until a land object gets thrown into the mix - the AI doesnt seem to take the land into into consideration)

I have seen a similar problem to your where the Russian subs will crawl/scrape along the floor (in a planes down position) trying to go even lower.

IS the warm welcome thing broken - I seem to get one a day

Currently for all ships, land doesn't exist... You will see some weird 'strandings' of ships if you play the single mission TLAM strike near Murmansk: cargo ships will happily head towards the land and be stucked there.

True, I was playing the single mission where you sink on the Kirov the other day and it ran aground before I even spotted it.

I also saw a Delta IV to a blow straight into the bottom of the ice pack in the last mission of the campaign. It just sat there pinned against the ice until I put another torpedo into it.

AI subs have vertical avoidance (depth changes) as part of their tactics, but this doesn't account for shallow waters and icebergs, that's the reason why they sometimes hit the ground or get stucked in the ice pack.

Lately I had a very interesing fight against a Alpha and Victor I sub, these subs have shown very good tactics evading my wire-guided torpedos, changing depth or going at max speed at the surface. At some point I had to use Sea Lance missiles to confuse them, after 2 hits there were still there, getting 800 feet deep - because of flooding - then one of them imploded, the first time I saw this happening. With shallow waters and icebergs the same tactics would be catastrophic for them, that's what you're seeing.