I can only comment based on the pictures - you can see it better than I can. But in the pictures, it does certainly seem to have a "dip" - right after if makes it's first 90* turn.
Also, the hose seems to run downhill - that part is fine if the thru-hull fitting is lower in the hull. But if the thru-hull fitting is higher, then that's another issue that needs to be addressed.
It would be logical to think that it might fill just fine to 3/4 sometimes, and other times it doesn't. It all depends on where/how the water collects in the vent hose and if/how much the vent fitting is clogged. Even if you saw water spurting out of it, it could still be somewhat clogged. Meaning, when you fill the tank to 3/4 it may initially bulge, but then it starts to go back down as the air slowly escapes.
Also, the hose seems to run downhill - that part is fine if the thru-hull fitting is lower in the hull. But if the thru-hull fitting is higher, then that's another issue that needs to be addressed.
It would be logical to think that it might fill just fine to 3/4 sometimes, and other times it doesn't. It all depends on where/how the water collects in the vent hose and if/how much the vent fitting is clogged. Even if you saw water spurting out of it, it could still be somewhat clogged. Meaning, when you fill the tank to 3/4 it may initially bulge, but then it starts to go back down as the air slowly escapes.