I have a '72 30 MK1.
When I purchased the boat mine did/does the same.
My boat's hull was barrier coated, but not the keel and paint was flaking off the keel.
First I torqued my keel bolts. Spec's are on CNCPhotoalbum.com
I stripped the keel, primed, barrier coated it.
I then taped of a section around the smile.
Using 3m 4200 (I think, it was years ago) and a putty knife, I covered the taped off area, making a flexible "gasket". Thus the smile might return but water would not get in under the barrier coat.
Next I barrier coated over the "gasket", not sure why, but I did.
10 years later I'm still doing just fine.
A smile does sometimes return but I simply paint over it. The 3m is still flexible.
I think the smile is due to the leverage effect when the boat sits on her swept back keel.
If I knew now what I knew then, I might not have bothered with the "gasket" but I guess I can't argue with results 10 years later
Any questions, please feel free to PM me.