A lot of time has passed and a lot of versions have been released since I reported this issue, but it still happens.
I don't remember if at the time it occurred in the exactly the same way it occurs now: in 2017 I wrote it happened after zapping to whatever 4K channel.
But, what's for sure is that (at least now) the message appears after leaving a 4K channel, it doesn't matter if you zap to another 4K one or to a HD or SD one.
So, it doesn't appear while you zap from a non-4K to a 4K channel, but only after the following zapping, wherever you zap to.
So, if the 2nd, 3rd, 4th zaps are all to 4K channels, you read it, until you zap to a non-4K, when you get it for the last time.
The timeshift is set to my internal HDD and it is formatted not as ext2 or ext3 but as ext4. But I think it should not happen, as ext4 is the fs image coders advise to use.