Can't upgrade T2 affix despite potential AND shards?

Working on this helm, I have forging potential remaining, but I cannot even click on the T2 Armour affix to upgrade it, nor will it let me add an Armour shard to the middle circle.

It’s not a level issue, since I can’t even select it to get a “you need level XX to forge” message, and further, I’ve upgraded other (higher) affixes since. I understand that I can’t upgrade the bleed-penetration affix, since I have no shards for that, but I have 52 armour shards and (most recently) 11 Forging potential.
Why can’t I upgrade the T2 Armour?

Screenshots:

Player.log (408.4 KB)

Is it because it’s increased armor and not added armor?

It’s % armour, and the shards (shown as “cannot be applied”) tooltip also says % armour

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