Mine is the same - it is to rinse out the pump / sump before taking in the
main fill of water and detergent. without this stage it could be possible
for the initial inrush of water to flush the detergent load straight into
the sump where it wouldn't mix properly.

So does that mean it's normal then?
I put a bottle of Mr Muscle down the sink, but it didn't make any
difference. It doesn't seem particularly slow at draining normally
either.

I was wondering - does anyone here use a detergent that is quite foamy
at 40?
Formil sounds good - especially if it works at 30 and is kind to your
skin.