GPS metadata lost on save of PNG file

Latest (as of October 20, 2017) versions of Photoshop and camera raw, all available Adobe CC updates applied.

I am losing GPS metadata - and ONLY GPS metadata - when I save a PNG file from Photoshop. This has only started happening recently.

Steps:

- Using ACDSee add GPS latitude/longitude (and other metadata like IPTC content) to a raw (NEF) file. Data is saved in a XMP sidecar file.

- Open NEF file in camera raw, do some edits, open in Photoshop.

- Save as PSD - the GPS (and other metadata) is saved in the PSD file.

- Do some edits, saving to PSD as I go.

- Do a final Save As to a PNG file (not Save for Web or Export - just a normal Save As). All metadata, including GPS, is present in the PNG file.

- Open the PNG file (which definitely has the GPS data at this point) in Photoshop, do some minor edit (eg: levels), save (not Save As, just Save).

- GPS data is gone from the PNG file, but all other metadata is still there - ONLY the GPS data is lost.

- Say some bad words, put the GPS metadata back.

So the GPS metadata is only lost when I open an existing PNG file in Photoshop and save it.

Seems awfully specific to only lose the GPS data. I suspect there is something "helpfully" stripping it out. I could not find any preference setting for this. What am I missing?

David