Did you ask for a change of team / change of work focus? Did you bring up your issues with a manager? If you were on my team and an attrition risk due to not working in the tech you were hired for I'd have made sure that was changed.
Five months is just about enough time to find your feet in a large org's codebase, I think having exposure to the Rails side is to your benefit, and given a bit more time you could have worked to resolve the other gripes. Shopify is a huge, profitable organization, with many, many, many opportunities for developers to do their best work. I find it hard to believe there isn't something fascinating there for anyone who likes writing code.
Good luck in whatever you are doing now though, sounds like you have the talent for doing whatever you wish which is a great place to be in.
FWIW I do not work for Shopify.