I too am experiencing this issue. 4 total contacts in group message (1) Android. Really thought it was an iPhone issue and tried changing the iPhone settings multiple ways and none of which worked. Could send video to Android user only with no other contacts in message. Also once on WiFi was able to send as intended To all 4 contacts in one message. This makes me think it is DEFINITELY a Verizon carrier issue.
I have several group chats and would be concerned too if I were having any issues with sending messages. Are you having problems only when sending a video message? Or does this happen with standard text messages as well? What version of iOS do you have installed on your phone (Settings >> General >> About >> Version)?
I have been having this issue for the past 2 years. I'm an apple user.
I upgraded my iPhone 6 today to the iPhone 13 thinking it was an Apple issue. Apple tech support looked into it and are saying it's a carrier issue. I went to the Verizon store today and they were quick to tell me that it could be the android user's issue but when I told him it happens with all my contacts who are android the Verizon tech got flustered and told me to wait to find out. Reading the posts on this thread suggests that it is a Verizon issue and it's time to end my 20 year customer loyalty with Verizon and switch carriers if they cannot fix this problem.