Completing a tower with 6 modifiers doesn't unlock 6 mod waystone slot

I completed a tower using a waystone that had 6 modifiers on it, and this did not unlock the 3rd slot for waystones. It also does not appear to work in the preview when you are putting the first waystone in (before completing) either. I even tried putting a waystone with 7 modifiers on it, and that also does not show the correct state when previewing.

As of now, I have no idea how you are actually supposed to unlock the 3rd slot on these towers, because what the UI says is completely wrong. What's up with this?

Some mods are hybrid mods (that is, 1 mod on 2 lines). Before using the waystone, you want to hold ALT on it to see how many mods it actually is. When you hold ALT, there is a bracket on the right side for each mod. You want to make sure there are 6 different brackets.

I should also add: A 6-mod waystone only allows 1 attempt (so 0 respawns allowed).