Per the datasheet, the T3 is fully protected against ingress of dust, and against full immersion in shallow (<1m) water for less than 30 minutes. That's the verbose way of saying it is IP67 rated.
Going RTK scuba diving?
Regardless, I wouldn't expect a plastic bag to have any significant effect on GNSS signal reception.
For reference, my LS+ (also IP67) has had no issues playing in the rain.