Had a lone rider(tranformed) first strike kill a metalwork colossus that had been reinforced and lone rider ended up dying even though it had way more power. Maybe the colossus breaks apart from its trigger or something?
If your Lone Rider just turned into It that Rides that turn, there is a known bug about it. Basically it deals only the damage that it had as a lone rider, and not it that rides. It’s super annoying. I’ve lost games because of it. 
I had it that rides out already as a 64/12 and a lone rider as my 3rd creature to reinforce it.
Yeah it’s almost certainly a problem with Lone Rider // It That Rides As One. I recently gave 13/6 Voltaic Brawler double strike and it killed a colossus with first strike damage and had no problem taking damage on the return.