If a Microsoft System Center Configuration Manager distribution point (DP) from a different boundary group is in the same IP subnet as the client, that DP is used before the DPs in the client's current boundary group that is in a different subnet are used.
Boundary group relationships define fallback behavior that lets a Configuration Manager client expand its search from its current boundary group to additional boundary groups when it searches for an available site system.
The usual fallback behavior is to use DPs in the current boundary group instead of those from the neighboring and site default boundary groups.
The current boundary group is always intended to be searched first. However, if a DP from a neighboring boundary group is in the same IP subnet that a client is in, that DP is used before the DPs that are in the client’s current boundary groups that are not in the same subnet.
To avoid this issue and make sure that the usual fallback behavior occurs, do not define neighboring boundary groups to be in the same subnets as the clients in the current boundary group.