I'm getting some unusual interaction between filter layers and the layer opacity setting and wondering what's going on.

First, explanation of the scenario and structure:
In a file I'm working with, I have multiple layers that are a mix of two colours, blue and black, and I've been using a filter layer ("Color to Alpha") to selectively hide the blue when I want. The layer structure follows:

• Layer Group
  
  • Color to Alpha Filter
  • Blue+Black 1
  • Blue+Black 2
  • etc.

This works as one would expect, removing the selected colour (blue) from the blue+black layers in the same group. Nothing strange so far.

Then I noticed that the filter layer's opacity is adjustable, and got an idea. I lowerered the layer's opacity and it did what I expected, making the blue lines reappear, but much lighter. Great! Exactly what I wanted: to selectively lighten the blue marks without completely removing them, so that I can see them but only just barely.

Then things got weird. After a change, the filter layer's opacity slider greyed out and I couldn't change it again. Selecting another layer and back makes the filter layer's opacity adjustable again, but changes it to 100% (or whatever the opacity of the last-active layer was), though the canvas itself isn't changed and the blue marks are still visible at the same opacity. Saving and reopening the document in this state results in the filter being back at 100% opacity and the lines completely hidden.

I tested with a simpler structure, too: a single layer with blue+black lines and a filter mask attached to it. Same thing happens there.

So, now I'm curious about what's going on here. Changing opacity of the filter layer works, at least with this specific filter (and a few others I tested, like invert and desaturate), but Krita itself acts as if it's unexpected/unwanted behaviour, and even refuses to save the change. It seems like there's a bug here, but I'm not sure which part is the bug. Should it not allow filter layer opacity changes at all? Or is the bug that it's not preserving those changes and the UI is blocking multiple adjustments?

This is happening in the current krita-testing package in the Krita Lime repository. No idea about other versions.

I was hoping somebody would have some insight on this, but it's been a couple weeks with no bites. Does nobody know what the intended behaviour is supposed to be here? Or was I unclear about something and nobody can tell what I meant?