A few things are possibly gong on. Are you religious about varnishing and have enough UV protection on the boat? I had an older build where that was happening and I was negelent about varnishing regularly. Plus you have to wonder how thick varnish should be for good UV protection and what brands do it best.

When I epoxy I used to wipe off the brush on an old outside table that I painted on etc. Here's what's interesting. The epoxy, totally exposed to the sun just gradually disappears. It doesn't blister, or get soft or any obvious break down - it just slowly disappears. That's what happens without adequate UV protection.

I am more cautious now with my boats and I no longer have weave starting to appear with regular coats of varnish.

I think this is just an educated guess because a host of things could be going on with your boat but I believe epoxy disappears without adequate UV protection. It must do something like evaporation ???