Yep, my 12LA bucket acts just like that when back dragging. Sometimes it flops like a rag doll - take another set and it holds. I'm glad to hear someone else has seen it happen. I was concerned a bit that I had hurt something when I built my oversize root rake that attaches to the bucket. I've given that loader quite a workout. In my usual casual manner, though, even when I thought I had hurt it, I ignored it -- whatever will be will be, and I have found I can work around it.
As for the rear end getting light, I was moving heaping buckets of damp sandy dirt with a box blade on the back for a counterweight. The back end wouldn't come off the ground (loaded R4s), but every bump in the ground gave me the heebie jeebies. Also, a couple of times when I dropped the bucket to carry it low, I dropped it a little too fast and jerked it to a stop - and got a stomach-queezy bounce. If the bucket was just heaped, it wasn't too bad, but on several loads I got a really heaping load - almost as much above the bucket as in it. I don't know that I saved any time with those loads, because they were scary enough that I drove slower to my dump spot (about 150' away).
My nornal technique is to use low range to fill the bucket and back away from the pile and turn around, then, at the same time I'm lifting the hydro pedal to go forward, I slide into high range and boogie for the dump spot. When the bucket was overfull, I stayed in low range and creeped along.
A couple of times I had an unbalanced load (higher on one side than the other), and, while turning around with the wheels cut hard, backed up onto a small rise or a tree branch. Now, that's a scary situation - feels like the thing is going to go over diagonally. Whoa!
That loader is incredible, and I think is the main reason why the little TC18 seems like it can do almost as much work as its bigger brothers.