The Grumpy Editor reviews Claws Mail

If 'gnus-verbose' is at least 7, you'll see messages of the form "nnimap: 
Setting marks in %s..." when the read flags are being updated.

It's saved by 'gnus-summary-save-newsrc' (Z s), on group exit, at article 
motion time, and at some other moments: it is *not* saved immediately 
merely because you read an article (that would be much too slow with some 
backends).