Describe the bug
There are many blocks that cannot be added to the filter, an example is iron bars(minecraft:iron_bars).

Conflicting mods
To follow the iron bars example, Quark. The thing gets compressed into iron grates(quark:grate)

To Reproduce
Try to place iron bars in the item slot of the filter.
The item does not go into the item slot, and remains in the cursor.

Versions
backpack 3.15.18.570
core 0.0.5.12
forge 40.1.15

closing the ticket as this works as expected

why should iron bars go in there? in vanilla they are 3x2 recipe and those are not supported - only 2x2 and 3x3

to blacklist or whitelist the block from being compressed in modded environments. Also...it doesn't go into ANY match tag filter.
Not even the pickup or magnet.

well there's nothing to blacklist if it can't be compressed by compacting upgrade so that's why it can't go in the filter and the reason it can't go in tag filter is because it doesn't have any tags in vanilla for sure. Unless of course the modpack you have adds tags to iron bars in which case that would be an issue that will require fixing, but the only condition that's used there is just a quick check to see if the item has any tags at all if it doesn't it can't be put in that slot.

So unless you have any additional info here all of this so far is working as it's supposed to and I am going to close the ticket soon

maybe I do not understand, but isn't minecraft:iron_bars the tag for it? like say.... forge:gems? or they are different?

minecraft:iron_bars is the item name. Tags are assigned to items on top of this to put them into groups of things that can then be referred to. So minecraft:diamond and minecraft:emerald have forge:gems tag.

so because they don't have a tag they can't be filtered by "match tag" :S?

well yes, because there's no tag to filter on. You can still filter for tag not being present which will match the bars for sure because they don't have any.
If there isn't anything else here I am going to close as this is working as expected