you should have done this first, ^^^^^ then taken it in, now its on record with clean fluid and no issues. kids dont listen. if you want a new one under warranty, kill this one first, then take it in. you did it backwards. you just bring it to them f ed up good. not a intermittent issue
my 03 gmc truck started to slip in reverse and first gear, you had to stand on the brakes and floor it to make it happen. shop calls me back says its fine, i rip the guy a new asshole and tell him how to diag a slipping tranny, then told him, ill stop my day right now and be there in 20 minutes and ill melt the tranny right in front of them if they cant figure out the issue.
it took 3 trannys to get it right too, two rebuilds, then a gm crate tranny, and a crate t case and a rebuilt rear end, all with 45 miles left on the warranty.
moral of the story is, make sure its " broken" when they get it the first time.