This is something between very minor UI issue or a small bug, and defenetly nothing that should fix fast.
On my forum I didn’t have tag-block on the sidebar. If I added one or more tag from sidebar settings of my profile it came visible. If I deleted tags the block disappered.
I’m using heavily tags and because of the Hub I can’t manually edit urls, so easy access to tag page is important to me.
I cleaned CSS edits, tried safe mode and rebuilded — no luck. If I didn’t add at least one tag at settings of my own the block was not visible.
Last thing I did was the one I should do very first: admin settings of the sidebar. When I added at least one tag as default, the tag block did Grand Return™. When I deleted that default one from my own settings, the block was still visible.
I count that behaviour as a bug, or at least an UI-issue, because it is not consistent with categories. But it is really minor one, because I reckon most of admins shall use default tags (and the big majority of users… just don’t care because they use searching instead).