Showers in house run cold after 30/45 seconds - Sinks scalding hot?
Wait, there's an expansion tank in the HOT water side?  That's wrong, it's supposed to be on the cold-water side of the hot water heater.  Expansion tanks do not have heaters in them, so the water in them is cold!  Try this theory on for size: You turn on max-hot, until the water runs nice and hot.  The lengthy hot-water pipe from downstairs is now full of hot water.  Then you set it for mixed-warm, which flows a lot more water overall, and reduces system pressure.  What does the expansion tank do?  Equalize to that new pressure - the air in the tank expands, by pushing cold water out of the expansion tank into the "hot" line.  It gets upstairs 30 seconds later. 

How do you test this?  First, when it gets cold, touch nothing - just let it keep flowing for 120 more seconds.  See if it goes back to normal if you wait long enough.  If so, I'd blame the expansion tank being on the wrong side.

Or check the tank's temperature - room temperature - then trigger the problem - and shut off everything and run back down and check the tank's temperature again.  If it's warmer, it just ingested some hot water - which means it recently pushed out some cold water. 

Otherwise, I'd do some other experiments.  Shut off the cold supply to the shower.  Now if it does the hot-cold bounce, it can't be the mixing valve!  Turn the tub to hot max.  Let it get hot and watch for that bounce.  None? Throw open the sink faucet to COLD and flush the toilet, to simulate the extra flow of setting the shower to "warm".  Watch for the bounce again.  If it still happens, that means the (good) valve isn't getting good hot water.

What would make it suddenly break? Probably something minor upstream that wouldn't affect healthy plumbing. Maybe loose cruft in your piping (probably upstream of the hot water heater). Maybe in street piping.  Maybe the water company changed supply pressure.