Is this the same issue as you mentioned in the other thread here?
In that case, the main difference was that 16.04 had an old version of snapd (2.34, since the newer 2.37 is stuck in proposed). And since installing a core18 based snap does not trigger installation of core, you don’t get the newer snapd delivered via that snap.
When the newer snapd is promoted from xenial-proposed things will likely sort themselves out, but in the mean time running snap install core should work around the problem.
Perhaps this should increase the priority of getting the new snapd pushed out to xenial users?