It is related to the elastomer resin being used.
In the elastomer resin, the particle structure of the material itself is large, and there is a characteristic that oil enters into particles.
Even if it is washed with a neutral detergent, oil that has entered the particle may come out after a while.
Since the oil that comes out is oil of the food, there is no harm to the body.
Please wash quickly so as not to get oil.
When the oil stain becomes severe, wash again carfully and do boils/sterilizes, you can reduce stickiness somewhat.
Depending on the oil that entered, even if you wash, the stickiness may not get rid of it.