Sounds very much like the torque converter is draining and needs time to fill up with enough fluid to work.
I'm not familiar with these boxes (only had a 3speed in the past), but isn't there a one way valve to stop this?
Definitely take it back to the place it was purchased from and get them to fix it!
My XF had a stuffed auto, so I got a reconditioned box and new torque converter (shift points and pressures modified for max performance). The first replacement TC played up doing similiar to what you have.. so I took it back and got it replaced. The second one was fine tho.
Crazy.. but an XF can do 90km/h held back in first.. and literally launch changing from 1 to 2.... gotta love a modified auto!
88 EA Falcon S
RIP my ol' esky.. many years of good service