The best thing to do is get information from your ISP. Perhaps they have a beta program or something that would result in more information.
You can see what PD you are getting by saving the DUID in System > Advanced, Networking
Screen Shot 2019-08-18 at 11.56.41 AM.png
Then enable the Debug mode on WAN in the DHCP6 Client Configuration area, setting whatever secret sauce your ISP requires. This is what I use for Cox Las Vegas:
Screen Shot 2019-08-18 at 11.59.41 AM.png
Your ISP might require something completely different.
Then look at Status > System Logs, DHCP and set the filter to process dhcp6c
You will see exactly what is happening.
My PD looks like this:
Sep 1 03:55:10 dhcp6c 44071 update an IA: PD-0
Sep 1 03:55:10 dhcp6c 44071 status code for PD-0: success
Sep 1 03:55:10 dhcp6c 44071 update a prefix 2600:dabb:ad00:bc00::/56 pltime=34359824768, vltime=34359824768
Sep 1 03:55:10 dhcp6c 44071 executes /var/etc/dhcp6c_wan_script.sh
Sep 1 03:55:10 dhcp6c dhcp6c renew, no change - bypassing update on igb0
Sep 1 03:55:10 dhcp6c 44071 script "/var/etc/dhcp6c_wan_script.sh" terminated
If you want to try new settings just increment the DUID-LLT, save, and Edit/Save WAN. That should result in a new renewal using a new DUID so it should all be fresh.
Your ISP might have settings that don't like changes like this. Only they know. Ask them. We cannot know what they require here. Again, only they know.