that doestn't look like butyl to me
it looks like a polyurethane than never fully cured, and melts
i saw a shower skylight doing that in an RV we looked at
it dripped on the sides of the shower stall
clean it up and seal it with something else, to keep it in
the only real cure is to remove the window clean out all the old OEM sealant
and reseal the window with something that will not do that
Butyl tape or maybe 3M polyurethane or pro-seal ?
I can explain it to you.
But I Can Not understand it for you !
....
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1997 F53 Bounder 36s