If metadata is added to PNG files, such as keywords, it doesn’t “stick” permanently. I found the only safe way was to use Bridge > Photoshop > Image processor and batch change the PNG to a JPEG, PSD, or TIFF file. I used tiff LZW to save space and even then it is a larger file than the png. I needed this to be able to add keywords and descriptions to pngs in scrapbook kits.
This was not my idea completely. I was changing the file extension in a more round about way and found this presented by a scrapbooker whose name and site I forget. I think her first name was Kayla.