I have had this a number of times - and sent logs to Dave. I am cleared to one altitude, reach it, then hear no more about changing my level, even though I have a 'nag' set for non-compliance (the nag works in other circumstances). Other ATC instructions, like headings, are OK. After a while I 'test' things by contacting ATC and get the pilot announcing '... out of flight level XXX for flight level YYY'.
I have the ATC log running too (currently scuppered by the fact, as reported, its slots fill up and then it shows no more, but I am sure Dave is on to that) and that, when there's still space, shows no instruction to climb either.