Green Thumbnails on video files

I have recently rooted my Google Nexus 7 (2012) and flashed it with Lineage OS / Android Nougat.
All is fine except the thumbnails of my video files have a green, mauve (or both) tint.
This applies to 99% of .mp4 files, whilst .wmv, .flv and one odd .mp4 files have no thumbnails-
just the standard camera icon.
This is happening irrespective of what file viewer I use. (eg the stock "Files" app or Astro)
Thumbnails of jpeg pictures are ok and unaffected.
Before rooting, there was no problem in...

Green Thumbnails on video files

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