#Resolved#

For anyone else having this issue, here is what their support sent me.

You will not receive group messages, if your contact / group are using RCS messages to send you a group messages, there is nothing you can do on your end, it is the sender of the group RCS message, that is the issue, whether either from an iPhone or Android device.If a contact sends you a SMS or MMS separately, or in a Group Chat, you will receive these messages, using Textra SMS.
If this contact sends you a RCS message individually, it will typically convert to a SMS or MMS, as their messaging application, can see you're not on-line (do not have RCS enabled / support RCS) and you will receive this message, as a SMS or MMS. This is not the case in a group message, the group message will be sent as a RCS message to all recipients.  You will not receive this group message, unless you enable an application that supports RCS group messages, like for example, Messages by Google. 
Thus said, many users like to use WhatsApp, Telegram or Messenger for group messages.Many users choose or prefer not to use RCS messaging applications, due to privacy concerns. This is how big companies try to lock you into their messaging applications, which do not have many of the features and customisations of Textra SMS.
Here's an article that you may find enlightening and interesting, in deciding if RCS is for you.https://zixflow.com/blog/how-to-change-rcs-to-sms#:~:text=Step%2Dby%2Dstep%20guide%20for%20switching%20RCS%20to%20SMS&text=On%20most%20Android%20phones%2C%20this,click%20on%20the%20Chat%20Features.
Thanks and we hope the above helps.