It sounds like your outbound was in the first 12 months of Megavolotea membership, and your inbound was just after the 12 months?!

A few posts down I have posted about how Megavolotea becomes Megavolotea Plus after 12 months, and the Plus means taking the cabin bag onboard.

They also get slated on trust pilot for their baggage rules which is a shame as they appear to be trying to be a bit more generous than their low-cost peers, but don't communicate it well.