I did a fresh install of Ubuntu MATE 18.04 on one computer, and an upgrade from 17.10 on another. The upgrade is missing the snap packages for Welcome and Software Boutique and neither shows in the Control Center. The fresh install has them.
Whether this happens to anyone who upgrades, I don’t know. I’m sure one could install them afterwards, however.
Yes, it’s mentioned in the release post, you have to install them manually. The upgrade process can only update packages from the repos.
BTW, there are three snaps to install, not only the two that you can spot easily in the Control Center:
snap install ubuntu-mate-welcome --classic
snap install software-boutique --classic
snap install pulsemixer
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pulsemixer did get installed by the upgrade. Here are the snaps in the upgrade:
dmn@Sydney:~$ snap list
Name Version Rev Tracking Developer Notes
core 16-2.32.5 4486 stable canonical core
pulsemixer 1.4.0 8 stable georgefilipkin -
Pulsemixer was already installed in 17.10 
(I think it's included in those instructions for people who upgrade from 16.04.)