Now here is something that will not work this way. This is Legio II going all the way back from Bibiracte to Italia. They did not replenish in Raetia & Noricum, not in Cisalpina and finally now they do not replenish in Italia. Still based on a very small empire, this means Legions will become incompatible to any historical expansion.
There must have been some kind of supply system. I do not have any historical knowledge on this system, but Roman roads have been built for many reasons... A total dependence on local population will not work for sure, especially as this seems bugged. Pops in the provinces travelled are bigger than 0, so at least there should have been some replenishment.