If PixInsight lost the capability to write WCS headers to FITS files, which is how I interpret the release notes, then that's a real step backwards. Not writing them in XISF files but use XISF properties instead is fine. It's PixInsight's own image format after all. Anyway, I will look into this.
Could you please temporarily share one of your images (e.g. using iCloud) so I can investigate the issue you are having with the new plate solver? I find it way more robust than the old one, but it always can get better.