Mms has this hability to answer all or not. this can be set in Textra setting (I use reply all mode because I want to be able to see all people that receive the Mms and also I want to send one Mms instead of several sms for a group of people.
but sometime I want to be able to answer only to the sender of the Mms. and it is not possible quickly as it can be done using email. the best would be to have a reply all option that would not be activated by default (security....). like email.