Question:
Wasn’t there a Second Front ruling where you pointed out that isolation was only checked at the start of the turn, whereas out-of-supply was checked twice, before movement and before combat?

Answer:
No, this is backwards. Isolation is checked more than once per turn, it specifically says so in the isolation rules. Supply is judged only during initial phases and that supply condition prevails trhoughout that player turn, regardless of changes to the local situation. It has always been so.

Source:
TEM 67 and TEM 76