Small engine (brushcutter) problem

Hi all:
Posted this to misc.rural a few days ago, but got no real answer, so I was hoping someone here might have an idea:
I've been using saws, cutters etc. for, oh, 30 years. By no means an expert, but I have used them in all sorts of weather, terrain, temperatures and elevations.
Yesterday, was using my Husky 232R brushcutter for a little more than an hour and was on my second tank of 50-1 mix when I decided to take a break. Swigged some water for about 10 minutes and walked back to the cutter and noticed that the fuel tank cap was shimmering with dampness. Looked closer and saw that the gas was "bubbling" out of the gas cap vent hole.
Unscrewed the cap and it opened with a "whoosh" of escaping air. Noticed that it kept doing this all afternoon. Now, of course it was warm. But I've cut in much warmer weather and never noticed over the years.
And, gas fumes escaping would be logical, but liquid fuel bubbling up? As I said, no expert, but never saw this before. Normal?
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Tim J. Johson wrote:

boiling the gas... probably the vent hole also needed some cleaning.. that would make it shoot out the hole in a pressure state???
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