apt list snapd
Listing... Done
snapd/xenial-updates 2.25 amd64 [upgradable from: 2.21]
N: There are 2 additional versions. Please use the '-a' switch to see them.
apt list snapd -a
Listing... Done
snapd/xenial-updates 2.25 amd64 [upgradable from: 2.21]
snapd/now 2.21 amd64 [installed,upgradable to: 2.25]
snapd/xenial 2.0.2 amd64
Thanks! I think the issue will go away if you apt install snapd (as a workaround) - we will work on a proper fix now, I think the deb version was the missing piece of data we needed. Thanks again for this!
Absolutely yes. Thanks for the help
This is the thing that makes it broken. 
So the 2.21 story is still broken but I think we now have a way to fix it cleanly.
We will have a branch that fixes this shortly. A new core snap should be in edge in a few hours.
The pull request is at https://github.com/snapcore/snapd/pull/3606
We did not build a core snap yet as the PR arrived late and was not reviewed. We will focus on it this morning.
A fix for the snapd 2.21 deb for the bug that @ramur encountered is now in the beta channel. So if you encounter this bug, please snap refresh --beta core and let us know if the bug is fixed for you.