What percent of the inside lining of the Instant Pot actually stainless steel?
There is no such thing as a mixed material containing "x percent steel", unless you are talking an inhomogenous composite material (which would be most certainly more expensive than using all steel, and wouldn't look metallic). A metal alloy is either a steel (stainless or not stainless), or it isn't a steel.

There are thousands of stainless steel types (which you could group by 3 major types and a few specialty ones) which have varying corrosion resistance. Also, a major factor here is how the steel was polished/finished.

Have you compared cooking the same dish in another (non instant, non pressure) stainless steel pot? 

Were there any other metal items (spoons, standoffs, immersion blenders...), especially made from something else than stainless steel, more than momentarily present in the liquid while cooking or while the contents were still hot? Stainless steel doesn't like being part of a boiling battery...

A salty, sour liquid at pressure cooker temperatures is actually a rather aggressive chemical mixture - even stainless steel has to work hard here.

By the way, the stainless steel typically used for pots has mostly iron (good for you), carbon (not bad for you), nickel and chromium (technically not good for you)...