I noticed it happening again in 7.0.9 and reported it.
The response was
It also happened on a brand new laptop when my old one died, so unlikely to be cache?This is more likely to be a caching error, but I will log it for the developers
The ticket was closed and I have yet to test it in 7.0.10, but is the only way to resolve 'Cache' to remove the Thumbnail file?
Seems an extreme thing to do for the less savvy Computer user?!