First try to export the event slots to a baroc file and check the details of the event, if you see the same information for mc_long_msg slot.
If you dont see this value in the mc_long_msg slot then it is because of the following reason which is also mentioned in a KB article - KA326442
Problem : -
"In BPPM user console, the 'Detailed Message' of event shows data. But if the event is exported to baroc file or propagated to another cell then the mc_long_msg slot is empty."
Solution : -
"This behaviour is by design. The Rate process does not attempt to fill in the slot "mc_long_msg" because it does not have the information. This information (and hence slot) can only be retrieved at a time after alarm has already been created and is usually done by the alarm rule engine (because the alarm rule engine is instrumental in retrieving the Detailed Diagnostic information).
RFE QM001676969 has been created for this."