I have run into this problem a while now, for past Photoshop versions as well as the newest. Only recently I found out why. The mistake is exactly replicable, I've done it multiple times and everytime it repeated. I did the following:
1. Open LR
2. Open PS and check whether a GPU is detected -> no
3. Close PS and try again
4. Close LR and try again -> GPU detected
I have done everything that I have seen posted on the community about GPU issues (update driver, set preference to GPU, etc.) but nothing worked.
The latest update is having issues recognizing mid-range GPUs. Try installing the previous version of Photoshop and see if the issue persists. Adobe is aware of this issue and is working on a fix. In the meantime, this is the recommended action.
Adobe Community Professional/Adobe Certified Instructor