This is my first post, hoping to explain well so that it's understandable.
Does Meraki have any option/backend feature to control routes advertisement between two MX250's setup in Active/Active single arm concentrator mode?
We have a situation where we are experiencing a split/asymmetrical routing for locally connected subnet at DC 1 to reach an IP of other MX250 in DC 2.
We are advertising subnet a.b.c.d to MX250 and to ISP MPLS core via BGP in DC2 so other MX250 at DC 1 is learning from core and also from iBGP built between two MX250. Traffic going from AP(teleworker VPN) to primary MX250 at DC 1 is going via MPLS and when it gets to MX250 at DC1, DC1 is seeing a shortest path through iBGP hence return traffic is coming via tunnel between two MX250's.
I assume your DCs are using some kind of layer 3 switch to do the core routing and that runs BGP to the MX's? If so - that is where you need to attack this problem. You need to configure the layer 3 switches to prefer MPLS routes over routes from the MX.