Sorry, I can confirm now it was a different issue. I did a brand new wordpress install and checked for any differences and realized what it was. AFC is working in 5.1 for me now.
If it helps anyone else, it was simply that the urls in my wordpress/.htaccess did not exactly match what was saved in the general settings.
I think that mismatch broke the save feature.
Hmm, I shot a screencapture right here showing its very explciityl the ACF having the issue. This is a clean brand new install of WordPress 5.1
Emnabling and disabling the plugin has obvious affects where the post no longer works.
https://monosnap.com/file/apoSahREEzcXKFpOX8JzsSuiKpxYOk
