Oh, I see the problem. SDDM’s last release is 0.20.0 from June 23. However, a few days later a fix got merged that replaces the .desktop file parser with a new one that conforms to the Desktop Entry Specification. The actual .desktop file is valid, but the last released version parses it incorrectly and a new release including the fix has not yet been released.
Looks like we’ll just need to wait until that happens, no further action or MR necessary.
Edit: I added a bullet point to the Plasma 6 known issues list. Hopefully people are okay with that, it’s quite a bit less severe than other bugs listed there, but also is immediately visible and prevents people like me from getting confused/concerned about it.