Photoshop Undo Not Working (Toggle Last State Only, No Undo)

I'm having an issue with using undo in Photoshop. I recently updated to CC 2019 and Mojave at the same time. I am familiar with the changes Adobe made to the undo/step back shortcuts. When working in photoshop, undo will randomly not work (typically when I need it most). I can use "Toggle Last State" once, but have been caught in a predicament where I flattened the file, moved it and could only toggle the move, not the flattening, so the file had to be reverted to the last save. There is no rhyme or reason as far as I can tell, and if I try to replicate the issue, it will work fine, then randomly not work. When it happens, if I go up to edit, undo and redo will be greyed out, and toggle last state is active. If I toggle the last state, I still cannot undo or redo. I did discover that if I do get stuck, I can go into the history and cycle back through previous iterations. It's not a huge problem, it's just aggravating that it's not consistent, and I didn't know if there was some user error involved.

Specs:

macOS Mojave 10.14

iMac (Retina 5K, 27-inch, 2017)

Photoshop CC 2019 20.0.0