I'll try to help, but I'm not sure about EVERY details of your fork, so let's see:
RLC means (adjustable) Rebound, Compression and Lockout.
If it rebounds, there is still some preload on your fork, but if your weight is enough to make it go down, perhaps the preload is not enough. Since your fork is air-sprung, it would make me think of very low pressure on the air-spring compartment.
Lockout works, so there is oil inside, which is the mechanism used by most lockout systems: entirely closing the compression oil valve.
You said PSI is at 90-100, which in some models is too little pressure. I have an RST model which demands me to put 170 psi. In the other hand I had another (larger bore) suspension which was fine with 40 psi. I don't know if this pressure is enough.
Even so, perhaps your model has MORE THAN ONE schraeder valve, so one would be "positive" pressure, the other would be "negative pressure" (or "low speed compression", or any other fancy name the factory creates).
The most interesting part is "it seems to SLOWLY sink when I'm on the bike". See questions later.
I have this questions, which you could put on comments or re-edit your original question:
- Do you have the manual, and what PRELOAD air pressure do they recommend for your weight?
- Do your model have more than one Schraeder valve? If yes, how are they named, and which one has the pressure you mentioned (where is it located)?
- While the suspension slowly sinks, this happens while it is working (absorbing bumps and rebounding back), or just statically applying your weight on the bike while stopped? If you pull it down half travel, does it come back bevore continuing to sink, or continues to sink from this half travel position?
Hope it helps