JaccoW

Sounds like a combination of galvanic corrosion and perhaps a bent bolt that now has bent the frame as well.
Stainless steel is very strong but it does 'work harden'. Meaning it will become hard and brittle when drilled or bent.

My first instinct would be to tap out the hole so it is at least straight again, maybe in a larger diameter so you can retap it.
Then always use lube to install bolts. You will thank yourself a couple of years down the road.

Also, if the insides are tapped, why not use two shorter bolts? Less risk of this happening or them bending and getting stuck. That's an awful lot of bolt to use.