My experience, regardless of insider participation or not, is once you get to an AU build, whatever was available at the time, you are stuck there unless you use WDRT to revert back to 8.1 and repeat the process all over again. This was true for me in these 3 upgrade scenarios...
This was all prior to the first RS2 builds which are not what's delivered on Insider Fast.
WDRT back to 8.1 --> Update Advisor gets me to 10586.xxx (whatever was available at the time) --> Insider Fast - 14393.5
No further updates so I...
WDRT back to 8.1 --> Update Advisor gets me to 10586.xxx (whatever was available at the time) --> Insider Fast - 14393.67
No further updates so I...
WDRT back to 8.1 --> Update Advisor gets me to 10586.xxx (whatever was available at the time) --> Insider Fast - 14393.103
which is where I sit now. Not sure what i'll do next but its far too time consuming to do this every time. Then there is the 'restore' which is always incomplete - the whole backup and restore process needs an overhaul.
Also not sure what will happen now that RS2 is on Fast, I assume maybe choosing Slow will get me the latest AU build, or with a little luck the UA will deliver the AU.
It is annoying.
It would be awesome of the WDRT prompted you which build you wanted to START with. Not sure why that isn't a basic function.