I've had some experience in regen braking on RTG cranes with diesel generator plant, where the resistive dump bank has not been sized sufficiently.
The net result is an increase in genset speed as the set goes into reverse power mode.
In one instance in the UAE, a 1500kVA D/G set went to 2050RPM during the 40 ton lower brake test.
The reverse power operation tends to cause high circulating current in the generator rotor damper winding. In this case the heating lasted only for around 40 seconds, but prolonged periods of damper cage heating will cause failure, usually ath the damper lam corners, where the section is at a minimum.
As others responding, I would suggest you add or increase resistive load banks to absorb the generated power and negate the reverse power problem.
Hope this helps