Rune Mending should also work on used up Runes

[February]

I mean in situations where you picked up the best rune for your particular ability (e.g. Primary Attack), have used it up completely, so that the red bar disappears, and then should be able to Mend it via the Mender NPC.

At least in a situation where you consciously have not picked up another one - think of it as a trade-off; you purposefully do not pick up a rune for a particular ability (e.g. Primary Attack) and can still Mend it as a reward of doing the rest of a Dungeon with -1 Rune in total.

The fact that you have to have at least a tiny sliver of the Red Rune Bar remaining on an ability in order for the Mender to Mend it is... well, logical on one hand, but irritating on the other. Even with all upgrades maxed out, there's often a situation where you find the best possible rune for your character and just end up running out of it before you reach the Mender. And because you haven't found the same Rune a 2nd time, you cannot Mend it.

In other words, Rune Mender should remember the last Rune you have used on all your abilities and still be able to Mend them, or rather restore from nothingness in this case, I suppose.