Yes.
This is incorrect. You're inadvertently referencing a path outside the jail. The jail has no concept of "/mnt/Pool1/....." Only the proper TrueNAS host has such access. You need to reference the path in the jail itself that points to this location. (This location, which you gave access to the jail via a mountpoint.)
See my previous post for an example.
EDIT: To help you see it from a different perspective, consider this:
Anything after /mnt/pool/iocage/jails/jailname/root
is where the jail's root filesystem begins, and hence, how it "sees the world".
Take this path on your TrueNAS system, for example...
/mnt/pool/iocage/jails/myapp/root/media/downloads
If this is the real path from the host's perspective...
/mnt/pool/iocage/jails/myapp/root/media/downloads
Then this is the path, as seen (and understood) from within the "myapp jail"...
/media/downloads
(Where mountpoints come into play is directing certain paths, within the jail, to have access outside of the jail.)