Use meld in Kate editor, Flatpak version, to compare files and find differences

In Kate editor I have been using meld to compare files and find differences. I was happily using the contextual menu that you get when you have two files open and you right-click on the tab of the non active document, as explained in this answer . However, after moving to the Flatpak version of Kate, this doesn't work anymore... I still have meld installed, and the "compare with active document" menu appears, when I right-click on the tab. However, I get a message: "The selected program can not be started. Maybe is not installed". I guess this has to do with the "sandboxing" of Flatpak. I have messed with some of the settings for Kate in "Flatseal" (the sandboxing manager for Flatpak), in particular granting to Kate access to all system resources, but I can't get meld back. Maybe I need to export an environment variable? but which one?