CWDT and self-cast skills

I have Arcane Surge linked to a high level self-cast curse in an item that also contains Cast When Damage Taken (Level 1), but Arcane Surge only works without CWDT in the item for some reason. It doesn't even increase the mana cost of the curse as long as CWDT is there.

EDIT
Spell Cascade and Spell Echo don't apply as well, Increased Duration does. It seems like support gems that don't work on triggered skills also can't apply to triggerable self-cast spells in a Cast When Damage Taken setup.

If this is intended behaviour, I'd recommend adding something like "triggerable spells get the Trigger tag, regardless of their level" to the description of Cast When Damage Taken. "This gem can only support skill gems requiring level 38 or lower" makes it sound like none of its effects would apply to anything higher.