According to Bill Hamilton (a semi-famous Vipassana meditator and teacher), the arahat Taungpulu Sayadaw chose to reincarnate and take the Bodhisattva bow, this was on the whim of his teacher, who wanted one of his students to reach arahatship and teach Vipassana, and his other student to take the Bodhisattva vow.
The former was Mahasi Sayadaw, the latter was Taungpulu Sayadaw.
The issue of masters reincarnating who have seemingly ended rebirth is dealt with in the Mahayana and Vajrayana vehicles or branches of Buddhism, it appears to be somewhat of a paradox, or "learning what you already know", so there's no real need to rehash it. (What happens is a snag is hit, and so the computer pauses, and delves deeper and deeper (what are these called? successive, delved, nested loops? iterations or stacks or whatever, check out recursive functions), into what it already knows, thus it appears like things are "happening", but time-space is really frozen, it hasn't moved in forever, and is hinged on some problem which it is unable to solve)
Even then, the Zen master Hakuin Ekaku, stated that the ultimateness of Awakening was none other than being a bodhisattva and teaching others!
This phenomenon is often demonstrated with individuals who purposely strip themselves of all attainments and enter the void again, to do it all over again. I think Christ is a good example of this (someone who emptied himself of Godhead, and made the journey all over again).
You might check out the Tulku phenomenon, and the seeming presence of already enlightened individuals, who merely recapitulate the journey, see: Adi Da Samraj.
I have a friend like this, he's somewhat mysterious to me
The other issue of an 8=3 backsliding into "black brotherhood" is a non-issue, because it assumes permanence, or a linear version of time (past, present, future), etc
Any attainment is not "permanent", because it has not happened yet, and so what seems like an "awakening" or "attainment", happening at some point in space-time, is really not that, but something that has already happened, intimating itself through an appearance of causality (it happening)
Thus there is no permanence "up there", nothing is sticky, and in the 4D space, anything can be rearranged, "down here" it appears like things are being lost
(the 4D space is a space where time-space is "frozen", and so things don't really happen, rather they are moved around, but down here it appears like things are happening, or are lost, maybe they are just being sorted)
But it has already happened
Think about how a pen looks if you shove it through a 2D plane, it looks like it's changing, yet to us the pen is unchanging, it is just one pen, it's being shoved through a plane
Similarly in the 4D space, "where everything that can happen, has happened", time-space is "frozen", yet this "already happened" (immanent, and absolute event, hence the subitism argument) intimates itself through a seed-effect causality-like happening, as if things are happening, or that there is a genesis
This is an illusion
So when an arahat reincarnates, he is not losing anything, because he has never had anything, all the pieces of the puzzle are still up there, but are being sorted in some realm wherein we cannot tell why they are there, so "the subjective appearance", may be that the arahat regains his enlightenment
Who knows though
93,
~JJ
Edit:
In 4D space-time things that "happened" in the past can be changed, so while it appears like there is "permanence" or that things can be lost, in reality the pieces are still there, but taken out of our awareness, and being rearranged
It's slightly creepy
So everything there is "frozen", or remains there, but we may not be aware of all those things
It's more of a space where there are a finite amount of pieces that can be changed and rearranged around, but to us it appears like things are "happening" or being reversed (the past being changed), etc
There is no "out there" (somewhat of a separate idea, relating to Advaita Vedanta)
Edit 2:
In other words, what has been gained, apparently, through time, can in fact be lost, because in the 4D space, it is not time (and thus, certainty, or, the past), but in fact a space, wherein things can be rearranged, so the past is not sealed, and nothing, happens
Edit 3:
Time gives the illusion that the past is immutable or sealed, but in a space, things can be rearranged, nevertheless it appears, to "happen through" time, and time is the medium of the intimation of the frozen object, which knows no time, and already is the case, we're merely getting glimpses of it
I guess that's why some people say time is cyclical