Cannot level up a duplicate skill gem

I have the same skill gem (Explosive Grenade) slotted in two meta skills: Mortar Cannon and Mirage Archer. When I level up the gem, it works with one of them but never the second one. Game even seems to acknowledge that there is something to level up (a + sign next to the skills in the skill popup), however it only shows that the gem will be leveled from the current level to the same one (i.e. 14 to 14), and if I confirm the selection, it eats the uncut gem with no changes made to anything. The unleveled gem stays the same level.

The only workaround that seems to work (since there is no option to use the uncut gem directly on the skill gem) is to remove both gems from the skill inventory, slot the unleveled gem into it own row, and level it up like that, then slot everything back where it was.