David Hansen wrote:
Connections should not be made to the
stack within 200mm of the centre point of a lavatory connection.
This is to avoid the possibility of the lavatory discharge blocking
or obstructing smaller waste pipes.

Instead make the connection at least 200mm above or below the
lavatory connection, using a suitable boss. Whether it is above or
below depends on your plumbing. The hole can be made with a suitable
hole cutter (into a plastic stack).

If the connection is made opposite the lavatory one, 200mm below,
that seems to be in the perfect position for being blocked! That
said it doesn't seem to happen IME.