The axis encoder is used for threading. Tacho generator is on end of motor. The encoder and the tacho signal probably don't agree, with the design limits.
Is it braking at the full current? It sounds like the full deceleration being commanded but is not happening correctly. In the LC15 there is a tacho generator on the end of the motor spindle. Very fragile wire in armature. Careful.
The tacho brushes: I found this were worn, and spindle control was erratic until the brushes were replace. Surface condition of commutator is important, and my problem was brush bounce. Is there any squealing noise from the tacho?
I found spare brushes in side the control cabinet. That was nice find
Reducing the speed in two steps might be a good workaround until you have identified the cause. I do suspect the tacho generator, if the LB15 has one, needs service.
6 or 8 years back since I was involved with it.