The current decription of Model::dirty indicates that the dirty information is available in all hooks:
* The $dirty data will be reset after you save() the data but it is still available to all before/after save handlers.

However, that is not the case. Model::dirty is reset before the Model::HOOK_AFTER_SAVE spot is executed (taken from current develop):

            $dirtyRef = &$this->getDirtyRef();
            $dirtyRef = [];

            if ($this->idField && $this->reloadAfterSave) {
                $this->reload();
            }

            $this->hook(self::HOOK_AFTER_SAVE, [$isUpdate]);

It is not enough to move the reset after the hook, as Model::reload() also resets dirty. This is where is gets complicated: reloaded model information could be important in the after save hook, as does the dirty information does.

The only fix I can think of is to clone $dirtyRef, assign in to $this right before the hook is executed and then reset it afterwards. Something like:

            $dirtyRef = &$this->getDirtyRef();
            $dirtyRefCache = clone dirtyRef;

            if ($this->idField && $this->reloadAfterSave) {
                $this->reload();
            }

            $dirtyRef = $dirtyRefCache;
            $this->hook(self::HOOK_AFTER_SAVE, [$isUpdate]);
            $dirtyRef = [];

WDYT about this? If you are fine with this, I will create a PR with a change like this and add a test.