There’s no bug; the lens effects will be updated again after some more render passes are computed. The lens effects calculations take some time and V-Ray doesn’t do them all the time in order to give more time for actual rendering. By default the interval at which the effects are updated is calculated such that the lens effects take no more than 10% of the total render time.
If we find a way to make the lens effects faster, especially for large resolution images, we can increase this default interval.
Best regards,
Vlado