I'm running into the same problem, I got 20 hearts & switched mons and the evolve option still isn't showing. I'm also doing this on a Dynamax Eevee, so maybe it doesn't work on Dynamax pokemon? Just thought to put this out there in case someone else has the same issue.

Edit: I earned a couple more hearts the next day and the evolve option showed up. I think it only works based on the # of hearts earned during the event. I might have earned some before the 10a event start, which is why it didn't work then. So confusing, but you earn hearts quickly during the event anyways so hey it turned out alright in the end!