Minecraft version
1.15.2 / fabric 0.9.1-205

Mod version and malilib version
litematica-fabric-1.15.2-0.0.0-dev.20200515.184506
malilib-fabric-1.15.2-0.10.0-dev.21+arne.2

Description of the bug
Schematic verifier overlay that shows glowing outlines on blocks I select in verifier used to work in some earlier releases (especially for missing ones), but now no matter what I try they don't show up anymore on literally the same schematic I'm still working on. (Glowing outline feature can been seen here: https://youtu.be/aMZSXAKZzpo?t=749)

Make sure you have that placement selected. Either click on it in the Schematic Placements list so that it has the white outline, or middle click with the tool while pointing at it in-world. In the older versions (ie. all the current 1.13.2+ builds still) the verifier overlay was only rendered for the selected placement. I changed this a while ago in 1.12.2 so that it now renders for any active verifiers (so that will come to all the versions when the next code merge from 1.12.2 happens).

Make sure you have that placement selected. Either click on it in the Schematic Placements list so that it has the white outline, or middle click with the tool while pointing at it in-world.

This advice actually solved my problem, thank you! Though it feels very confusing, I have only one placement created anyway (so it has to be selected by default in theory, why is it not?), there must be some error in case of placement not being selected, there is no way user can figure this out on his own, user just sees no feedback from the program.

If you accidentally middle-click on air when holding tool in any of placement-related modes - it deselects your placement.

Also if you remove another placement that was selected, you will be left with no placements selected. You could maybe argue that in the case where there is only one placement left in such a case, that should/could be selected automatically.

But personally I don't like that, since it would be the mod changing selections automatically, and in some cases (with the creative tool modes) if you aren't aware of it or don't expect it, that might even have dangerous consequences.

Warning box telling me "please select current placement" when opening schematic verifier will be very useful.

Though it feels very confusing, I have only one placement created anyway (so it has to be selected by default in theory, why is it not?)

When you create a placement, that newly created placement is selected. But like was mentioned above, there are three ways it can get unselected: removing a selected placement (leaving other unselected placements), accidentally unselecting it with the tool (middle clicking while pointing at no placement), or accidentally unselecting it in the Schematic Placements list by clicking on the currently selected placement, which unselects it.

I'm not sure how to really improve this, as all of this is intentional and the way it is supposed to work. You can select or unselect placements with the tool or via the menu. Basically the user just has to be aware of the fact that there is the mechanic of selecting one of the existing placements, and the two ways to do it, and the fact that the selection is highlighted with a white outline in the menu, and in-world the selected placement has a cyan color, and also the tool HUD shows the name of the selected placement, or if there are none then it shows that too.

Warning box telling me "please select current placement" when opening schematic verifier will be very useful.

This is true, and in fact the Area Editor already does something similar. I'll add this I guess. Although the change to allow multiple verifiers kind of complicates how that should work...