Hold the model nose up the next time it happens.  If a bunch of fuel dribbles out the muffler -- it may be true.

Where is the muffler line going?  Have you watched it to see if it fills with fuel when you connect everything up?  On my tanks I take the muffler pressure off of a point below the top of the tank, yet as long as I plug the overflow before I connect the muffler line back up (I fill through the muffler line) I don't see fuel siphoning into the muffler.

If you're using the muffler line for overflow, disconnect it from the muffler until you're all filled up, then reconnect.  Then maybe suck a bit out of the tank to fill the muffler line with air, to prevent siphoning.

I connect the muffler to the uniflow.  I use the muffler tube to fill, with the cap off the overflow.  I had side mounted Tower 40 with muffler well below the tank.  If I did not do things right, I would get the muffler full of fuel.  If, after filling the tank, I first capped the overflow and then reattached the muffler line, no problem.  If I forgot, and reattached the muffler line first, then putting the cap on the overflow would push fuel into the muffler line and start a syphon.