PiHole Stats increasing and decreasing erratically

Your debug log shows that you have valid IPv4 and IPv6 IP's assigned to the Pi and that Pi-Hole is listening on both IP's using the ethernet interface.

The IPv6 error shown is not a concern per the note that followed the error:

   ^ Please note that you may have more than one IP address listed.
   As long as one of them is green, and it matches what is in /etc/pihole/setupVars.conf, there is no need for concern.

   The link to the FAQ is for an issue that sometimes occurs when the IPv6 address changes, which is why we check for it.

From your debug log, these lines are contained in your configuration file, so IPv6 blocking is enabled.

  [2019-04-02 18:13:22.256 27410] Starting config file parsing (/etc/pihole/pihole-FTL.conf)
   ...
   [2019-04-02 18:13:22.257 27410]    RESOLVE_IPV6: Resolve IPv6 addresses
   [2019-04-02 18:13:22.257 27410]    RESOLVE_IPV4: Resolve IPv4 addresses