This is one of the best questions I have seen on this forum, and not "easy" to answer IMO, and have had many thoughts about this very thing.
I'll give it a try though.
A higher AR wing has a bit more overall lift, a lot less drag when at higher AOA's, and it will have a steeper lift/AOA curve at high positive and negative lift which should give more stability when you push it hard in tight turns, but on the other hand a low AR can be pushed even harder without stalling the rear wing.
BUT, in reality, I think they are just high AR because you CAN, there are no downsides because it is so much smaller than the front wing, and it looks good too
And better wake turbulence immunity.
I got a supersmall really low AR rear wing (I call it a stabilizer though, as it is not used as a lifting "wing"), and it works just fine.
Very lively and agile - but because I dont have a high AR one so small, but got two bigger ones, high AR, I can not compare as they ride very differently anyways, when different size.
I have had a really low AR small one too on my very first foil years ago, to complement a (back then) new really small/fast frontwing.
And I felt that it could give an odd "burp" now and then, quite rare
But on the other hand, it could also be my lack of experience back then, or that these wings were not cut/sanded perfect - so hard to say...
OR, if too low AR it has less span, and might be influenced more by turbulence or even air bubbles from the front wing or mast - thus this occassional burp ?
Whereas high AR stabilizers will not be as prone to this of course.
So my take is, that because there are no downsides of high AR stabilizers, one will choose this to ensure you dont have wake turbulence issues.
If you ask me, I woud guess the AR of the rear wing does not matter much really, apart from the risk that turbulence could affect more.
If an issue or not will depend on your very hydrofoil of course, maybe not a problem, maybe it will.
Above is all feelings and theory based on experiences and knowledge - but I can not say for sure
Just go try it out
PS: Is it your first hydrofoil you are going to learn on, or got experience ? If you dont, have some experienced ride it - you dont just go out and ride such a thing and you need to know if it is hopeless to learn on, or if it works great, or something in between