I have noticed that the latest LNER diagrams have been published on WhatDoTheyKnow. I was having a look at them yesterday and noticed that this is a booked working. Diagram DN505, a 5 Car Bi-Mode unit works Harrogate services all day (Harrogate-London-Harrogate-London). A note on the right hand side of this reads "COUPLED WITH DN607" which is a 5 Car electric set. The diagram for the electric set shows that unit as working to Harrogate and not being detached at Leeds.
https://www.whatdotheyknow.com/request/671851/response/1594927/attach/2/2020 DIAGS May 2020 FINAL.pdf?cookie_passthrough=1
Normally, a double 5 Car set would have six engines, three per unit. By comparison, a 9 Car set has five engines. In this case, there would only be four engines in the formation, and if that video is anything to go by, only three are used to propel the train. I couldn't hear any engine noise from the front unit at all, so I am wondering if the single engine in the electric set is left at idle to produce power for lighting, heating/air conditioning, toilets and catering only? Presumably three or four engines is enough to propel the train to the line speed (60 Mph?) with acceptable acceleration.
It certainly surprised me when I saw it diagrammed that way!