In the above versions...

Pushing a block over a beacon, which in turn pushes another block out of the way, instead of just swapping them over as you would expect, the beam flashes white during the transition.

I have noted that from 1.14.3 PR 4 a peculiar side effect occurs 65 blocks above the beacon, the white flash stops happening on the beam, this 65 is not world height related but beam length as a higher placed beacon still shows the beam flash:

A beacon placed at height 11, has the beam stop flashing white from height 76, whilst a beam from a beacon at height 14 will still flash white until height 79.

See attachments: