you have to drop the Traveler stack then right click and you will be able to have the GUI open
and the Traveler sacks on the floor, in then gui Traveler Sacks take the items
then pickup the Sacks and right click it you will be able to take those items as well

Essentially drop the Travelers Sacks while fast right click you will have gui open and Sacks on the floor
being able to drop

here how to do it

1. first put items in the Sacks
2. drop the Sacks while you right click at the same time
3. you will have the gui open take the items out and pickup Sacks that was drop and take those items out

Forge - 14.23.1.2555
Actually Additions - 1.12.2-r124

@Shadows-of-Fire i had compile and test your fix
and can confirm that it's not fix

EDIT: now with the new commit you made it's fixed 872e124