Yes, as mentioned, the native VLAN on the MX is existential to forward the BPDUs to the MS switches in this scenario. By running PVSTP on the Catalyst side, you create a loop through the combined infrastructure. It's not for all VLANs, but enough for the meltdown. My advice would be to configure the native VLAN as the Meraki Mgmt-VLAN with no other devices in there in the infrastructure on the left. But that will likely still cause inferior BPDUs. Can you use completely different VLANs on the old and the new side? That would give some extra separation when you only allow the needed VLANs on the MX trunks.
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