Thanks.
For posterity I thought I’d link some related threads of people with Sony Bravia TVs with the same or similar problems:
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Sony KDL 60R550A and thumbnails (Describes thumbnails automatically being generated by “playing” the video and getting the first frame)
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How to enable DLNA movie thumbnails for Sony 2017 4K TV, KD-43XE7077 (in US is KD43X720E)? (This solution did not work for me)
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Does Plex support thumbnails / posters over DNLA for a SONY 2013 KDL50W670A (Again, this solution did not work)
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Sony Bravia 2013 Profile (Admittedly I haven’t tried these fixes, but my TV is a 2017 model and this is for a 2013 model, so I’m not optimistic they’d help).
As you can see this problem has been going on for years, some people suggest that it might be because the Sony Bravia TV might be expecting the thumbnails in a non-standard format.