macos – What could cause windows to “swap” desktops?

I recently migrated to a new 2019 MacBookPro, 16″ … running 10.15.6 … now I notice something I have never seen with my old Mac with the 3 desktops I am using:

  • Desktop 1 takes terminals, IDE, …
  • Desktop 2 takes browsers
  • Desktop 3 takes my mail client, slack, …

Meaning: when I startup my mac, and start my applications, I place the windows like that.

What I have seen now repeatedly: desktop 2 and 3 seem to fully “swap” at some point. Not sure, but I just come back and find: mail client, slack, … are now showing up on desktop 2, and my browser windows are all on desktop 3.

Any idea what could be causing this, or how to prevent it?

(I am not running any fancy additional “desktop related” apps, the only “special thing” I am using is the contexts window switcher).