Software sources are behaving strangely when an app is available as Snap

[AsciiWolf]

This happens on the latest GNOME Software 3.34.2 on Ubuntu Focal. When an app is (also) available in Snap Store, its sources showed in the "Sources" drop-down list in window header on app details page are behaving strangely:

- When an app is available both in the Ubuntu repository and in Snap, default source displayed is "Ubuntu" although the actual details page displayed is for Snap version.
- When clicking the drop-down list and selecting a Snap source, the Ubuntu source disappears.
- When clicking the drop-down list and selecting an Ubuntu source, the whole drop-down list (so basically all Snap sources) disappears.
- Default source is displayed as "Ubuntu" even for apps that are not available in Ubuntu repositories, only in Snap Store (for example the powershell).

This was in GS log, however I am not sure if it is related to this issue:
12:23:31:0206 GLib g_str_has_prefix: assertion 'str != NULL' failed
12:23:31:0215 GLib g_variant_new_variant: assertion 'value != NULL' failed
12:23:31:0215 GLib g_variant_new_variant: assertion 'value != NULL' failed
12:23:31:0215 Gs not GsPlugin error g-io-error-quark:35: Invalid string value converting to GVariant
12:23:31:0270 Gs not handling error failed for action refine: Invalid string value converting to GVariant

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.