I have an inactive's planet that I had plans to take on my planets list, and I have previously worn it down to 5 population (with a little help from legionmates) in preparation for a sab. I used a variety of planet debuffs to keep the population low. When I alerted it again today in an attempt to get it sabbed again, I used an Ion Storm Charge and Engineered Virus on the planet, and when I went to attack I found that the planet had been instantly brought down to 5 population again, without any attacks (last attacked approximately 1 day 12 hours ago). This kept it from being sabbed (because it wasn't recently attacked), but more importantly approximately 500 population just vanished as it instantly reverted to 5.

Summary: Using artifacts on and lowering a planet's population and waiting over 24 hours and then using artifacts on it before attempting to take it again caused the population to be reduced to 5. The owner not defending the planet in any way may be related.

Have had the same problem. Planet was also owned by an inactive player.