It's a known system limitation where casting bonuses don't apply to spells you get via dropdowns in class features, as opposed to via Manage Spells. As a workaround, you can customize the signature spell to add 2 to the To Hit Bonus.
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Helpful rewriter of Japanese->English translation and delver into software codebases (she/e/they)
I'm not seeing the bonus from robes f the magi on signature spells selected at level 20.
I've specifically selected Vampiric touch and see +13 for casting with a spell slot right next to +11 for casting as a signature spell.
Swapping the spell for fireball shows one fireball with a save of 19 and another save of 21.
https://www.dndbeyond.com/profile/Sparketh/characters/63731064
It's a known system limitation where casting bonuses don't apply to spells you get via dropdowns in class features, as opposed to via Manage Spells. As a workaround, you can customize the signature spell to add 2 to the To Hit Bonus.
Helpful rewriter of Japanese->English translation and delver into software codebases (she/e/they)
That's unfortunate, but thanks for clarifying.