Had to ask because it looks white in the picture and you wouldn’t have been the first.
The thing is, you are limited to possible answers without taking it apart. I can’t think of many options other than a piece of grit in one of the mating surfaces, oil in a place where there is RTV and it has stopped the silicone sealing properly or a tiny nick or grit in a seal. Maybe someone else will have another idea.
Removed the pan today and could see where the oil had gotten through some of the RTV, so fingers crossed that’s all it will have been. It was just a bit thin.
Spent the rest of the day scraping off all the old gasket (what a nightmare) and getting the pan back on (also a nightmare on the car just up on jack stands ) also took the opportunity to give the pan a good scrub and paint whilst it was off.
Top tip by the way, I got some old long 10mm bolts and cut the heads off them, put one in each corner of where the pan would line up on the block as a make-shift alignment tool. Worked well so thought I’d share
(needless to say I just removed them afterwards and replaced with the corrects bolts