As said many times group messages are converted into MMS messages. Turn off MMS in settings to stop this. It happens on iPhones and android phones as is a feature of the software.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion.
This is not Customer Service so we are unable to help with account specific issues.Currently using:
Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)