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I solved this. My filename is long so I didn't see in the filename save as dialog that there was a .pmdx extension. This appears to not change even if I've selected to save as .xlsx and it appears that the filename extension has priority over the save as selection. Manually changing the filename end...

Attempting to save spreadsheet with .xlsx extension results in .pmdx being saved. Planmaker Professional 2018 (974.0203) 64 Bit on macOS 10.15.3.