If the battery is almost fully charged, and too much voltage is being produced by the regen, the battery will be unable to safely absorb the energy being generated without the possibility of sustaining damage. In order to prevent the battery from being overcharged, the Battery Management System
(BMS) should disconnect the cells when the battery voltage reaches a certain limit, thus protecting the cells.
With the battery now disconnected, there is no resistance to produce the regenerative braking effect and the cells, which are no longer being charged, will slowly decrease in voltage until they fall below their safe maximum limit, at which point they are electronically reconnected and regen will start to work again.
This cutting in and out of the regen should not happen so much when the battery has been used for a while, but you may find it still occurs, but at much higher speeds when the high current being generated may still cause the voltage to rise too high.
Similar problems can also happen with poor battery connections.
Alan