I have Gimp 3.0.4 installed via Flathub. My install seems to keep updating every onece in a while with the same version number 3.0.4. What is being changed to casue an update with the same version number?
Flatpak works a little different than the most package managers. There’s not really “one version” since it consists of the application & its dependencies (which are frequently updated in case of GIMP). The version is just cosmetical metadata.
You can check the abridged log via flatpak:
flatpak remote-info --log flathub org.gimp.GIMP
or the complete log via the Flathub repository of the application:
A few applications are not build by Flathub but directly by the application developers, e.g. Firefox. Check the Manifest in the Links section of an application on Flathub.org.