I too experience this issue constantly on group messages (which are sent as MMS). Also on Verizon using a Galaxy S8. When this issue occurs, I am also unable to receive or send group messages via Textra or the stock Samsung Messages app.
I've tried reinstalling, rebooting, keeping mobile data on while on wifi (developer settings) and nothing has worked. I've reset my phone to factory yesterday and have been using the stock messages app since, without any issues.
My plan is to use the stock messages app (although it is terrible) over the next couple days to see if the issue occurs. If it does not, I'm going to give Textra another shot and see what happens.