At trueachievents forum user DrSchlepenstein has clarified it to me
In that image what you have equipped is a Medal of Summer. That is not a Medal of Seasons which is why it isn’t working. In the corner of that image though you will see the Badge of Seasons, you need 3 of those to make a medal.
If that is the current Guild Event medal (I haven’t played Gems since the last one), then when the event ends, that Medal of Summer (and any other Tokens/Badges of Summer you have) will convert over to a Medal of Seasons (and Tokens/Badges of Seasons). You will permanently lose the Medal of Summer as they are only valid during the event, but once the event ends you will have a Medal of Seasons.
It will unequip if you are currently using the Medal of Summer when the event ends (as it disappears/converts), so you’ll just need to go back into the Medal menu and equip the Medal of Seasons then. At that time the achievement should pop.