If you're using Flathub, the flatpak version there always lags behind by a few days after a release. I'm sure it will be updated in a day or two. Give them time, as they've been very busy fixing bugs. 1.21.2 required quick fixes, and 1.21.3 only existed for a few hours, so those versions never made it to Flathub. 1.21.4 should show up soon.
According to the wiki, you can install flatpak versions directly from Anego. I've not done so myself, but that version is reportedly updated much quicker than Flathub.