I don't have problem per say just looking for possible root cause.
I have an 87 Ti log and the AIS needs to be cleaned of carbon every 7K miles or so. Which is a 10 minute job. When I purchased the car new, the idle was set to 950 rpm and everything was fine. I decided to readjust the idle to the factory 800rpm as per the FSM, I noticed that the bypass airscrew was set to 950rpm rather than the standard 600rpm, this of course was with the AIS closed. 98.6% of the time the AIS works as expected when clean. Now here is the strange part, if I drive at 50 mph for 10 minutes straight and then need to stop at a light. The revs commonly drops to 6-700 which lead me to believe that once I am cruising for the bit the AIS closes. This is most noticeable with a high electrical load due to winter conditions...lights, heater fan as you would expect. I realize this is easy to fix just bump up the air bypass adjustment and I am done.
My reason for this message to understand why the ECU would close down the AIS entirely.

my guesses.
Perhaps only a log AIS thing as the ECU does not know the position of the valve thus it close. The ECU should know the 88+ AIS position since it is a stepper motor