After installing the Plasma iso from the Full Sail blog post, all of the default system applications (such as System Settings, Dolphin, etc.) are missing their icons. There is an icon pack called SolusSC that seems to be the default, which is missing most icons. Installing the Papirus-Adapta icon set and switching to that fixes the issue.
As a note, I kept my old /home partition (which used the Solus Budgie defaults for icons) instead of nuking it all. Could that possibly have a bearing on this issue?
EDIT: Forgot to mention that after installing I did a full update, and the issue persisted.