It appears that the most recent software update 8H3UUWU0 has broken group messaging on the TCL FLIP 3.
In the Messages app, there used to be a setting to enable/disable group messaging under Options > Settings, but that setting is now completely missing after the software update. I can still text multiple numbers at once, but the messages are sent individually, not grouped.
I have already tried a factory data reset without success. Does anyone have any other suggestions I can try, or is there another software update forthcoming?