The xdg-desktop-menu script, when installing a menu entry, sets the application as default handler for MIME types that are listed in its desktop file. This behavior was introduced in commit 1f7f2ea7 with the following message:
> Make application the default handler for a mimetype if it is the only
> handler for that mimetype so far.
In fact, this is incorrect. xdg-desktop-menu does not check if there are other handlers or not, it only checks that there's no *default* handler already set. Even if it performed such a check, it would be strange to set the only possible handler as default explicitly.
I consider such a behavior is a misfeature. Is is not documented anywhere, and it is unexpected that the tool, the only purpose of which is to add a menu item, adds some MIME types associations. Furthermore, it is impossible to disable this (e.g via a command line option), and, as already reported in #71825, such an association is not removed when removing the corresponding menu entry or even when the application itself is being removed.
Please consider reverting commit 1f7f2ea7 and disabling such a strange behavior.
I agree with your assessment that it is a misfeature