As an aside, here's why there's two appdata shares
By and large, the majority of apps within the Apps tab have a path mapping of /config (you can see it via Show More Settings / Advanced View when editing the app). Any path mapping of /config is automatically set to whatever the OS has set within Settings - Docker - Default Appdata Path.
But, a small subset of applications do not have a /config settings for various reasons. One of these is piHole which has 2 path mappings (/etc/pihole and /etc/dnsmasq.d) Because neither of these is /config, the OS takes the host path as-is (which is on piHole set to /mnt/cache/appdata/...) This results in your situation of having the 2 separate folders.
Note that even though the template directly references /mnt/cache, in the event that your system does not have a cache drive (or in the event of when 6.9 is released, the cache-pool the template references), then CA automatically adjusts the templates to match the user's system
If there is no cache drive present in the system, CA will adjust the template to automatically directly reference an installed drive (eg /mnt/disk1 instead of /mnt/cache)
If running 6.9 and a directly referenced cache pool does not exist, then CA will adjust the template to reference a cache-pool that is installed, with a fall back to an installed drive)
These under the hood changes prevent the circumstance that pops up intermittently where the appdata for one of these few apps winds up being created in RAM instead of on a drive.