It would have detected that there was already an EFI partition if you was installing Manjaro after Windows and asked if you wanted to use it.
I'm curious about that EFI partition on your storage drive though, did that disk originally have Windows on it? It must have had an OS on it at some point if you created an ESP for it. You said before, sorry it's like 3 am.
I'm under the impression that both OS's are using the ESP on the second disk, and that the ESP on sda is just a relic from past adventures. Maybe install grub on the ESP on sda if it isn't being used for anything else?
Oh so there are two on the nvme disk, now I'm well and truly confused. See earlier point about it being 3am.