So I make my cakes with Marshmallow Fondant and always have this problem.
I start with a thick trim around the edge of my cake then add the filling (strawberry) I try to always roll my fondant 1/4 thick but it happens that I get some spots thinner. Then I cover my cake with buttercream. (just a thin layer. Last I cover with fondant.
Here's the problem... later I see strawberry spots
.............what am I doing wrong?

Please help. Thank you

Is the "thick trim" thick buttercream piped around the edge (a dam) of each layer before you add the filling? and are you going over the edge of the dam with the filling?

Dont' spread right to the edge either. The pressure of the layer will do that. And do you let it settle for several hours (preferrably overnight)?

I agree with all the comments try making a thicker dam and fill in the center. Spread out but not too close to the edges of the cake, the preasure from stacking will make it even. Also only other thing I could think of is a thicker crumb coat, especually at the seams (middle where both layers connect) and letting it chill a bit.