It could be the dipstick is not going in all the way?. It is OK to operate so long as it is above the minimum level hot. Too full will cause foaming and earlt fluid oxidatin and tranny failure. Also a little water in the fluid will cause this. I suggest a drain/refill and check the old fluid for water just to be safe. The old stuff is probably oxidized in any case if it really has been foaming.

A quick test for water is to put a teaspoon fullin a aluminum muffin liner and heat it with a flame. If it crackles as it gets hot--that's water popping (just like water in cooking grease pops)