I've been nursing my ancient Rebas along for a while now with regular services to keep them feeling good.
I've noticed though that even with 150psi in the fork there is 5mm of virtually free movement at the bottom of the stroke. Not completely 'loose' and if I was to guess it was still damped, just no resistance.
I think Solo air Rebas had a habit of bleeding air from the positive chamber to the negative but I have no idea how to release it, the usual advice is to somehow poke a zip-tie down the side of the seals but they seem too tight?
There should be a valve inside the air shaft. Remove the bolt and have a look. That should release the air in the neg chamber.
I'm not sure that's what's causing the dead travel though. There should be a bit of give when you pull the fork, about 5mm would be right, but it shouldn't be that easy to move.
I had an issue with my Solo Air Rebas a few years back.
The cause was a scratched air shaft that was allowing air to bleed between positive and negative when it shouldn't have been.
The symptoms were that the fork slowly hardened and crept down (I think) into its travel and it definitely felt like a bag of spanners around the sag point/first part of the travel.
The other thing my vintage of Rebas (2015) suffered from (iirc) is clogging of the transfer port.
Not sure if that helps your issues or not but it's what I can share on the workings. I caved in and chucked them at TF Tuned and in less than a week I had properly functioning forks. I suppose that's the point really... If you need to keep them running it's probably worth biting the bullet and paying someone who has seen the issue before and has spares sitting on the shelf to cure them and then keep up the home servicing.
Yeah, am a tiny bit annoyed as the issue developed not long after getting them serviced last time! The bike sits in the garage for long periods as I mostly ride the gravel bikes so it hasn't been used much, perhaps 300-400km.
I guess it's still worth having them looked at again, could try and get the Poploc working again while I'm at it!