The subject line is true (Dnet is reissuing keys), but may be easily misinterpreted. It appears that the keys being reissued are the missing ones they don't expect to be returned, ever. Their graph shows that from 4 days out to 40 plus days, only 75% of the issued keys have come back.
They know which ones come back, therefore, they feel they can safely re-issue that 25% group of keys without concern of duplicating work. This is why we are getting so many tiny, tiny blocks of work. It is very rare to get a 32 work-unit block anymore. Most are of the 2, 4, but less than 8 unit size. Very inefficient, but it will fill in the holes.
So, if you are running a pproxy, keep your input buffer small. If you buffer more than 10 or 20 days worth, it is almost certain that DNet may reissue some of the older work units. If the new recipient turns them around faster than you, you have wasted precious CPU time.