UDP multicast and unicast on same port : unexpected behaviour?

Question

When binding 2 event handlers on 2 different UDP sockets on the same UDP port, one configured for multicast, one configured for unicast, both event handlers are fired up when a multicast packet is received, which is not expected behaviour. When an unicast packet is received, only the unicast handler is fired up, which is expected behaviour.

What could be wrong in my code ?

package require udp
package require logger

set log [logger::init m5140]
proc logger::tree::m5140::stdoutcmd {level text} {
        variable service
        puts "\[[clock format [clock seconds] -format {%Y%m%d-%H%M%S}]\] \[$service\] \[$level\] \'$text\'"
    }
${log}::notice "[info script] Started"
set group 224.1.5.14
set port   5140

Unicast 
    proc mcastEvent {chan} {
        variable mcount
        incr mcount
        lassign [chan configure $chan -peer] host port
        set data [read $chan]   
        if { $data ne {} } {
            incr mcount
            after idle ${::log}::notice [list [format {multicast (%08d) from %s |%s| } $mcount $host $data]]
        }
    }
    
    proc ucastEvent {chan} {
        variable ucount
        incr ucount
        set data [read $chan]   
        lassign [chan configure $chan -peer] host port
        if { $data ne {} } { 
            incr ucount
            after idle ${::log}::notice [list [format {unicast (%08d) from %s |%s| } $ucount $host $data]] 
        }
    }
    
    ######################## MULTICAST ###################################
    # Create a listening Multicast socket
    set msock [udp_open $port reuse]
    ${log}::notice "[info script] mcast_port $msock"
    chan configure  $msock -buffering line -blocking 0 -encoding utf-8 \
                -mcastadd $group -remote [list $group $port]
    
    ######################## UNICAST ###################################
    # Create a listening Unicast socket
    set usock [udp_open $port reuse]
    ${log}::notice "[info script] ucast_port $port"
    chan configure  $usock -buffering line -blocking 0 -encoding utf-8 
    
    
    # Start multicast event handler
    fileevent $msock readable [list mcastEvent $msock]
    
    # Start Unicast event handler            
    fileevent $usock readable [list ucastEvent $usock]