Looks like @manuq isn’t keeping the nightly repo updated. So for now you have to build and install the flatpak locally if you want the latest features. You’ll have to clone the repo and navigate to the flatpack folder. I pasted the Readme from that folder down below.
The files in this folder are used to generate nightly Flatpak builds of MyPaint.
Stable Flatpak builds of MyPaint are on flathub.org.
To locally build and install nightly MyPaint using Faltpak:
flatpak remote-add --no-gpg-verify --user local-repo ${PATH_TO_REPO}
flatpak-builder --repo=${PATH_TO_REPO} ${PATH_TO_BUILD} org.mypaint.MyPaint.json
flatpak --user install local-repo org.mypaint.MyPaint-Nightly
Set PATH_TO_REPO and PATH_TO_BUILD to somewhere in your home. The directories will be created by flatpak if they don't exist.