Split Cards Fail to be placed into Exile by Plot triggers (e.g., Lilah, Undefeated Slickshot)

Came across an incorrect interaction between split cards and triggered abilities that cause those cards to become plotted.

I specifically observed this with Lilah, Undefeated Slickshot combined with either Invert // Invent or Expansion // Explosion, but it would probably also happen with something like Fblthp, Lost on the Range.

When casting either half of one of the split cards, Lilah correctly sees that the player has cast a multicolor spell and the trigger to put the card into exile as it resolves goes on the stack. However, upon resolution the card does not go to exile (and therefore does not become plotted) and instead is put into the graveyard as though the ability had not triggered.

I suspect XMage may be trying and failing to exile only one of the halves of the card, instead of correctly exiling the entire split card.

Per the actual game rules, the split card should be exiled and either half allowed to be cast from exile as a plotted card (regardless of which half was initially cast).