Sounds like a pretty uncommon sort of malady... I'm not sure I buy you're diagnosing it correctly.

See, Shimano shifters work on a sort of ratchet mechanism, so it's hard to envision them slipping. They have a common fault where the lever won't catch on the mechanism to turn it, which is due to one or two little pawls getting gummed up. But the spring that actuates the pawl which holds the mechanism in place is generally much, much stronger and there's pretty much no way gummy grease can stop it doing its job.

But then again, although they all operate off the same principle there's a lot of variation in Shimano shifter design, particularly at the upper end...

OTOH I've fixed dozens of Shimano shifters, road and MTB, and I've never come across the symptom you describe.