I have 2 pairs of MX with several ports connected in a pair of stacked switches and it has been working without any problem.
LACP & Link Aggregation
The MX does not run LACP or any link aggregation protocols. Connecting aggregated ports to the LAN of the MX is not supported; all connected ports should be un-aggregated. If multiple ports are connected to the MX from a single switch for redundancy, it is highly recommended that you run STP on that switch, to ensure that one of the redundant ports is safely put into a blocking state.