Hi, can you please upload and image? my Photoshop 2018 opens perfect any png files
Check that you have all the latest System and Photoshop updates installed and latest devices drivers installed on your machine. I have nor problems with png file on my Windows workstation.
It sounds like you are not using the latest version of Photoshop. There was an issue with png on the Mac that was fixed with Photoshop release.
The current version is 19.1.3 r293 (use Photoshop System info to check)