On 06/29/2020, I was doing dailies in Pandaria and I started seeing corruption effects. I had been 90/90 with no effects. I had not equipped any new gear. When I looked at my Character Info screen and clicked the eyeball, I was at 129 total corruption and 90 total resistance while I was in Pandaria. I added up all my corrupted gear while this was happening and it was totaling 90. But the eyeball on the Character Info screen still showed 129. However, when I finished dailies, I ported back to The Seal and everything reset back to 90/90.
As you fight Black Empire mobs, you will be afflicted by two stacking curses. One that increases your haste and resource costs that lasts for less than a minute. The other increases corruption and lasts a half hour. Likely as you ported into the Seal, some friendly mage or druid decursed your corruption debuff.
That’s not it. I know the debuff you are talking about and I get it every day. This was something totally different and hasn’t happened again since I made my report.