Thanks for reporting! Sanguine Signet not working with Griddle is intentional. It boosts healing from collecting restore cells, but since Griddle prevents those cells from spawning, it ends up having no effect. Some accessories will pair more effectively with Griddle though, such as the Omnishard.
I've tested it on Fern, and it does work, though Fern's healing mechanics can definitely be confusing. Fern gets 50% extra healing from restore cells, so instead of the usual +20 stamina per restore, she gets +30. For overheal, restore cells only heal for half the amount (rounded down). So if Fern is at full stamina and collects a restore cell, she'd receive half of that healing instead.
Oh, that would explain why it is difficult to keep track of how much is getting healed with the Sanguine Signet! I don't think I had internalised the overall complexity! Apologies!)
Gotta admit that Fern is my favourite even though I'm not sure if some of the others aren't more efficient for others jobs or even overall! :)