I have the same problem.
I don't have ADHD.
I have an internal sensitivity problem instead and some executive dysfunction most related to self regulation and internal processing.
As much as I want to prefer the same things, do the same things, having all that predictability...
A part of me just kept swaying elsewhere because of boredom, pain, or some other excuses generated by my head that just boils down to 'not in a mood to'.
It's like no matter how detailed, how far from the present the schedules are and what will happen, I'm not predictable.
Hormonal cycles, maybe?
It can disrupt consistency if one had to 'adjust' to every other hormonal state longer than it changes again, and even changes one's preferences.
Emotional inclinations can cause a lot of personal preference inconsistencies of the day.
The inability to ignore such (or any internal) factors would end up plenty of ignored schedules or impulsive planning that either leads to regret or not following through later.
Interoceptive hypersensitivity as a problem.
If one doesn't process emotions fast enough, don't recover from sickness fast enough if cannot be ignored and do the planned activity even for just a minute.
Especially with inconsistent metabolic internal cycles related to eating and sleeping...
In executive dysfunction, all that internal disruptors and so much in need of rapid transitioning.
Like every small thing counts instead of a small thing is a part of something larger -- the smallest of shifts counts counts equally and it drains plenty...
I cannot say if someone else has ADHD on top of their autism.
Only that I had questioned this case myself too often as an autistic.