I admire the work you have done for this and I am trying to help.
You are telling me that rarely the bug will randomly disappear but only for one play session? I'm assuming you have tried different areas of the map. If only you could identify the variable that changes when this bug doesn't happen, that would likely give us the cause of the bug.
Since you have rebooted and even reformatted and reinstalled we can assume that this likely isn't a simple memory issue. I'm also assuming a few things like the save file that it happens and doesn't happen is the same save file. I'm also assuming things like you tested this both connected and disconnected from the internet, with headphones on, headphones off, mute on, mute off, hdr on, hdr off, vrr on vrr off, ect.
Using the scientific method we would try to list and document any potential variables and test each one. In game things, like what time of day, if certain scenery is visible and specific location of testing will need to be defined as well. Also physical variables like the ambient temperature.
Thinking of what could cause this, just off the top of my head I would assume resolution or something within the game that scales up and down as the game is being played. Perhaps there is an overload of effects that is causing a queue on some of the resources causing popins specifically in the grass. Many games have a foliage setting that can be adjusted and can have major performance impacts. Perhaps they simply have the foliage up one level too high for the average PS5 in their settings behind the scenes because perhaps their dev kit could handle it but some PS5s cannot. Have you tried playing on a 1080p or 1440p monitor to see if the issues persist? Perhaps this would offload resources enough from the console in order to prevent the error. That isn't a fix but if we could verify that fixed the issue then we could present this to Sony and CDPR for further review. Sorry I'm late joining this and hope to see it be resolved.