The answer appears to be related to
setfile. If one runs
Tomass-MacBook-Pro:tmp tomasgallucci$ getfileinfo /tmp
created: 07/30/2016 14:32:36
modified: 01/16/2017 14:56:52
In my case, the bundle flag was turned (capital letters mean attributes are turned on, lower case means they're turned off). To resolve the issue I was able to run
setfile -a b /private/tmp
And I could once again navigate to
/tmp in Finder.
What set the Bundle bit? I don't know. But life wouldn't be worth living if there weren't at least a few mysteries.
Here's a Super User article that discusses the topic in some detail.