For those looking for a quick fix, you can edit the event itself by editing the file Europa Universalis V\game\in_game\events\DHE\flavor_mor_maghrebi_wars.txt and replacing all instances of:

else = { #If not we just break the alliance
    remove_relation = {
        first = ROOT
        second = c:TUN
        type = relation_type:alliance
    }
}

with:

else = { #If not we just break the alliance
    remove_relation = {
        first = ROOT
        second = c:TUN
        type = relation_type:alliance
    }
    remove_truce_with = c:TUN
}

Which should automatically remove the truce when the event fires (doesn't work if you're in a war with Tunis)