This is the natural grain of the wood coming through.
The ink and clear have absorbed into it like sponge. This is exclusive to the wood which is why you won’t see that in the inserts.
It is a real tough thing to predict.
The playfield likely looked as slick as glass when it was fresh and over time the wood has reclaimed the materials.
Sanding and polishing will help.
Reclearing will reset it but there is no guarantee that it won’t look the same in another 6 or so months.
I have redone ones like this numerous times over the years. Sometimes it can be fully eliminated others it can come back in time although never as bad.