Forget it... I know this answer.  I've already fetched the previous url in GSC and it shows as redirected. Google is seeing it properly but for some reason still showing it  in SERPs.

Yes Nicholas, I've done all of that.  As I stated above, the New URL is in Google's Index and has a few links to it naturally.  Yet, months later, we are still seeing the old underscored url, that 301's, show up in the SERP.

I have to believe this is a bug with the google index vs individual google's page valuation relative to the link graph.  It's as if Google is confused because the legacy, underscored URL has all of the link equity but Googlebot isn't seeing the 301...

God, maybe that's it.  How do I know if Googlebot is seeing the 301?

We created a more keyword friendly url with dashes instead of underscores in December.  That new URL is in Google's Index and has a few links to it naturally.  The previous version of the URL (with underscores) continues to rear it's ugly head in the SERPs, though when you click on it you are 301'd to the new url.  The 301 is implemented correctly and checked out on sites such as http://www.redirect-checker.org/index.php.

Has anyone else experienced such a thing?

I understand that Google can use it's discretion on pages, title tags, canonicals, etc.... But I've never witnessed them continue to show an old url that has been 301'd to a new for months after discovery or randomly.

Forget it... I know this answer.  I've already fetched the previous url in GSC and it shows as redirected. Google is seeing it properly but for some reason still showing it  in SERPs.