Your topology is doing exactly what RSTP and classic STP does. So switch 1 is the RB so that one will have both links in designated state and forwarding. And all other switches will have equal cost on both links towards that RB. But since sending port 48 is numbered higher, port 47 will be the Root Port FWD and port 48 will be ALT BLK on the other switches. So to fix your problem you have to make sure switch 1 port 47 goes towards the primary MX. I myself however prefer not connecting the uplinks of access layer switches directly to an MX but going through a CORE switch stack which allows for better east west traffic. If both MX'es are in the same room you can also connect your MX directly to your CORE stack and then only two links i total would be blocked.
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