I moved your post to a new thread so I could address it directly without continuing with the older thread.
Looking back at the report I submitted to the team before on this, it appears that the team updated how the UV effects were calculated in a previous update and that reports of this issue could be a result of that change. There were also reports of SPF meters dropping too quickly for players so I wanted to ask a few questions to confirm your experience and then I will pass it onto the team to look into.
Regarding the canopies, standing directly under them will not always block the sun depending on the angle of the sun to the canopy. It should be roughly in line with the shadow cast by the canopy but can you confirm for me that when standing under it during the day you are able to find a spot where it blocks the sun, UV is reduced and SPF starts to fill up? Let me know if this is tricky to do too as I can pass that feedback onto the team also. And thank you for describing your raft layout - this is very handy to have.
Regarding the UV and SPF meters themselves, are you finding they’re giving accurate readouts based on your previous gameplay experience? If you feel the UV is staying at “High” in places or times where it shouldn’t, or your SPF meter drops far too quickly, let me know and I’ll inform the team.
As you mentioned you were resuming a playthrough on a different slot, how old is this save? Could it possibly have been made in a previous version or was it created recently enough but you took a break from it?