No it is not usual , once it is clamped it should stay that way no matter how many rolls you unload
First is the clamp a single or double ?
How big around are the rolls ?
Are clamp grip pads going at least halfway
On rolls ?
Did this issue just start. ?
If it holds for several rolls then after a while of use start dropping the rolls I would look at the hyd safety locks
They may be bleeding off causing to
Leak down and losing grip
Or a cyl issue
One or both cyls may have a little bypass
That can cause an issue
I usually reseal cyls and replace safety locks
As a safety measure