That's not correct. A mulitline message has multiple line starts. \A matches beginning of string, and there is only one beginning of string in a string.
If the event were generated by a multiline codec you could remove lines using a gsub filter. Use an anchor then a character group that excludes newline followed by a newline. Something like
mutate { gsub => [ "message", "\[8mha:[^
]
", "" ] }
Not sure if that applies to events generated by filebeat.