This is because the mirrored and flipped in construct is using negative size, I fixed it before once, but then it caused alot of problem to other people due to its inconsistent nature and it makes things awkward for several people who use it. So it might be a bug to some, but I really prefer to avoid the negative size checking for now.
That thing aside, LiteTween is perfectly fine with mirrored or flipped object if you set the target to negative value.
If you need to tween a mirrored or flipped object, please take into account these things.
1. The target width could be negative for mirrored object
2. The target height could be negative for flipped object
3. If you tween starting from a non mirrored to a mirrored one, remember to set 'from current' or 'from last recorded' according to your need.
There are alot of other people using this for mirrored objects, so if I changed how it worked, it might force them to change those events and breaks alot of project also.