LOL...It would have been more useful to have had one of the .png files selected at the time of that screenshot so that we may see its metadata but never mind, try this anyway.
My initial thought was the usual 'filesize' issue and having tried it, albeit in Windows 10, it does replicate the issue you are having.
This isn't to say this IS the cause in this case but worth investigating.
Check the filesizes of those .png files against the preference in Bridge.
Use EDIT > PREFERENCES > THUMBNAILS and check the option on the right side of the dialog, 'Do not process files larger than...'. If this value is smaller than any of the .png files then it isn't processed, no thumbnail is created for that file and you just get the default PS icon.
You will need to purge the thumbnail cache for changes to take effect.
( [In Bridge] Select everything, Right click a file, and choose 'Purge cache for selection'...)
It may be worth purging the cache before anything else anyway, it could just be that simple, could be.
Failing that, another screenshot with a .png file selected, (so that we can see the metadata for it), may throw some more light on it.
Regards.
MrToM.