Mismatched desktop/login backgrounds (wallpapers) when changing "picture aspect"

[Rudxain]

 * Cinnamon version 5.4.12
   - Please specify if you are using the daily builds PPA
     (https://launchpad.net/~linuxmint-daily-build-team/+archive/ubuntu/daily-builds). No
 * Distribution - Mint 21
 * Graphics hardware *and* driver used (mostly irrelevant)
 * 64 bit
 * Attach ~/.xsession-errors, or /var/log/syslog (irrelevant, this happens all the time, every-time)

Issue

When the BG is an SVG, the scaling may go wrong in the login screen.

When using a solid color (or gradient), the old BG is kept on the login screen.

Steps to reproduce

SVG bug:
(I'll add an example SVG later)

Solid/gradient bug:

1. Open the Backgrounds app
2. Select any built-in BG to apply it
3. Go to the Settings tab
4. Set "Picture aspect" to "No picture"
5. Close all apps to see the desktop
6. Logout and see the previously selected BG as the user BG

Expected behaviour

The desktop and login screen wallpapers should look identical, regardless of image format and config

Other information

It seems the lockscreen uses a weird mix of "spanned", "stretched", "scaled", and "mosaic". It doesn't exactly match any of the "picture aspect" modes

Configuration info, if applicable

None

[Rudxain]

It seems the SVG bug is a slick-greeter issue. I'll close it here (for now)

The "matching colors" issue seems more like a feature request rather than an unintentional bug, because of system limitations