In the answer to the question Why is Shmita only Rabbinic Nowdays? it quotes the Rambam as requiring that each tribe be settled on their tribal plot:
משגלה שבט ראובן ושבט גד וחצי שבט מנשה, בטלו היובלות--שנאמר "וקראתם דרור בארץ, לכל יושביה", בזמן שכל יושביה עליה: והוא שלא יהיו מעורבין שבט בשבט, אלא כולן יושבים כתקנן.
Once the tribe of Reuben and the tribe of Gad and half the tribe of Menashe were exiled, the Jubilee ended, as it says "and proclaim liberty throughout the land unto all the inhabitants thereof" when all its inhabitants are on it, and they cannot be mixed up tribes with tribes, rather each sitting as appointed [in their tribal plots].
All the tribes were originally intended to settle on plots in the land of Israel. How is it possible that Yovel / Shmita etc depend on all the tribes being settled on their allocated plots in the Land of Israel, when in fact some settled on different plots outside the land of Israel east of the Jordan river, not the plots they were originally intended to settle on west of the Jordan river.
The laws of Yovel / Shmita had to have been given at Matan Torah, but the 2 1/2 tribes only decided to settle outside the land of Israel on the east side of the Jordan some 40 years later. I know the division of the land only occurred after the land was conquered, but the issue is that a requirement for them to sit on their appointed plots would clearly seem to have meant their appointed plots in the land of Israel.
So it seems to me that either biblical Shmita was never in effect, because all twelve tribes never sat on plots in the land of Israel, or that this can not be a requirement for biblical Yovel / Shmita.
Does everyone agree with Rambam on this issue and what are Rambam's sources for his claims.