As I recall, a hit by a Morgul blade in the heart changes the victim into a lesser wraith, a slave of the Nine.
Frodo wasn't hit in the heart, but a splinter was left behind. That splinter had the same effect on Frodo as the ring, but much quicker: fading him.
Because Frodo hadn't used the ring very much since the last time Gandalf had seen Frodo, it's most likely the difference has been caused by the splinter of the blade.
Jellinek
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