It looked very turbulent in that pic above and very much like the water level in the overflow was too high.
That's the only way bubbles can get into the tube. There will always be some, of course, but if, as you say, it is not overly bubbly then you need to increase flow through the overflow box by upgrading your return pump (or turning it up if you have that option). I am flowing 150(ish)gph
through my Eshopps 300 overflow. At around 100gph it starts to accumulate bubbles in the tube as the syphon just isn't strong enough to push the bubbles through.
Had a big bubble in my tube a couple of nights ago. Though to myself, "what the hell!". For ten months not a single issue and now a big bubble accumulates? Found the suction cup from my ground probe had come loose and was sucked up against the intake of the return pump. Removed it and the bubble was purged within a few minutes.