eBGP Loop with ACI Leaf switches

[BertiniB]

Hello,
I am trying to connect Cisco ACI Leaf Switches to an external network through a Meraki MX75, but I am encountering several problems. As soon as I enable the interface going to the second Leaf switch, some sort of loop occurs and we start having a CPU spike and packet loss. Does anyone have any idea what may be happening?
Interestingly, if I add an L3 Switch in the middle, this doesn't occur.

EDIT: The MX75 is not configured with Routed-Interface (only the Leaf switches), the MX75 has each interface in a dedicated VLAN.

[RaphaelL]

The MX is in concentrator ( ONE arm )  mode ?

[BertiniB]

No, it is in Routed Mode.

[cmr]

Do the leaf switches not require a spine switch on the same site (as in the left hand diagram), or am I completely misunderstanding this setup?

[BertiniB]

The spines are connected to the leaf switches, they are just omitted from the diagram. In the left-hand diagram, the switch in the middle is a common L3 switch, not the spine.