You go to the plugins page of your WordPress site, click deactivate, the page reloads but the plugin is still active. How does that even happen?
We have encountered this situation twice over the years. In both cases the cause appeared to be some other rogue plugin. Disabling the problem plugin caused the operation of our other plugins to return to normal.
Why that happens and why one plugin appears to be able to inadvertently interfere with other plugins remains a mystery. There is no easy way (afaik) to figure out which plugin is responsible. Finding it requires some trial and error which tends to dull the desire to carry on investigating once you have resolved the initial problem at hand.
I would however suggest looking at when each of your plugins was last updated by the plugin author. Many plugins have a “view details” link on the plugins page. Clicking that typically pops up a dialog. You will find information about when the plugin was last updated on the right hand side. Today our problem plugin said this…
Last Updated: 4 years ago
Right, perhaps not such a big surprise that there was a problem there. Note to self, keep an eye on how long each plugin we use goes between updates. If they get too venerable, find a replacement that is being actively maintained.