The site can't handle very well if you select mutliplt options. After de- and reselecting the checkbox "hide what I already own" it worked again.
Maybe it depends on the order in which the boxes are selected, who knows. 2 checkboxes usually work fine and are also enough. DLCs for my game + hide my owned junk + sorted by discount highest one first = success.

Of course this is no way around the "collections where I own all stuff" problem.

They could fix it. They could have this basic functionality that's expected from an online games store on day 1.

Yes, they could. But I'm not holding my breath.

Then again form the point where I am coming from (software developer, probably the same as quite some others here) this is not a bug in the first place. It's working as intended. One bought the single components, but not the bundle that contains all of them. The thing in question are the intensions, not the functionality.

At checkout the store recognizes that and gives a warning. Sure, they could do that in real time or even mark all the bundles as owned that were bought in components. The latter however would cause problems when it comes to refunds. One can't refund the bundle in any way to start with. If he refunds the soundtrack, then he must also lose the bundle.
Yes, all this can be implemented and I would very welcome it.

I am just happy that currently it's working better than ever.

just bought stuff and feeling guilty but happy they added the filter