This morning I noticed updates. I clicked on the panel to update, Discover launched, and instead of updating, it marked everything for removal. I then had to use KSysGuard to kill Discover. No matter if I "marked" or "unmarked" the updates, results are the same. It marks all for removal, and then I have to "kill" Discover. Repeatably.
Any thoughts?

I had some similar problem, but it could be resolved.

When Discover offered its new KDE application packages,
it told me beforehand, that it would remove one package.
I accepted, and Discover installed the updates flawlessly.

But then I checked the result with apt on the command line.
And here the conflicts appeared.

'apt' told me that 4 packages were autoremovable. So
I tried this. 'apt' said it couldn't do this.

So I did 'apt full-upgrade'. 'apt' told me 4 packages couldn't
be installed but one other package could be removed.

This didn't work well. Whatever I tried with 'apt' there were only
package updates that couldn't be installed. Even while Discover
previously didn't show any problems.

I therefore used 'aptitude' for package management.
'aptitude' did its duty. Afterwards there were no complaints
neither from 'apt' nor from 'Discover'.

Maybe You should try first 'apt', and if it doesn't work, then 'aptitude'.

I tried that. Didnt work. I actually let Discover go through with what I thought was "removal". It didnt update, upgrade, or remove. I opened "Muon" found the upgrades, and accomplished it that way. Updating via cli gave me nothing but errors.
This bug def needs to be fixed.