However, hard as it may be to believe, it would appear that the dust and wiper seals are now so good that they are effectively air tight, so the lowers are also a sealed system. Since the volume of the lowers changes as the forks are compressed they act as another air “spring”. There are two things that can happen here. First, as in my case, the pressure in the lowers can increase with use (for reasons I don’t understand) and prevent you getting full travel. Sticking a zip tie down between the seal and the stanchion can release this pressure and restore travel. Second, if you do the zip tie “trick” at full compression or have the forks compressed when you assemble them, you can have a negative pressure in the lowers, which will cause the forks to suck down.
So, it seems to just be an unintended consequence of improvements in the dust and wiper seals. The solution would be to put a pressure release valve in the lowers, which fox apparently do with their 40 series forks, but that’s not common.
I hope that’s some help to somebody and I’m sure that, if there are errors in my understanding, somebody will be along soon to correct them.
To my understanding that’s spot on and yes symptoms can be similar to the blocked port issue.