I believe it works like this: The Forecast tag should follow the same order the tag uses generally. Tags follow project order if the tag’s actions have never been manually sorted. If they have been manual sorted at any point (which, it seems like yours probably has been), it will follow that manual sort and any new actions will be added to the end of the list in the order they receive the tag.
I’m not sure if there’s a better way to do it, but you could make a new tag, migrate your Forecast tag items over to it, and make it the new Forecast tag.