After implementing "degrowth in developed areas" I got at least four pop-ups for the same areas going less developed, then more developed, less developed again, etc. This happened over several planning cycles. Kind of annoying from a gameplay perspective.
What could be happening is that a region falls into the "low-income" category, so the degrowth shrinking is no longer applied, and then it grows back into the "high-income" category, thus kicking in degrowth shrinking again, thus bringing it back to the "low-income" category, etc.
What could be happening is that a region falls into the "low-income" category, so the degrowth shrinking is no longer applied, and then it grows back into the "high-income" category, thus kicking in degrowth shrinking again, thus bringing it back to the "low-income" category, etc.