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Yeah you’re totally right that a heavier gauge pulls the neck towards the body and generally would make the strings sit further away from the fretboard.
Another reason perhaps is that newer strings tend to sound brighter and sharper and can make fret buzz seem louder, so perhaps you had some fret buzz before but it’s more noticeable with fresher strings? This has happened to me more than once.
Brb omw to Spain and their colonies