It would indeed be unusual to withdraw one from service because one of the engines was out. I have seen several running in service with one engine out before, and as Broadgage says power output increases in the remaining engines. Perhaps there was more to it than that?
The switch from
TOC▸ supplied daily maintenance to manufacturer supplied daily maintenance has some consequences.
We have already seen the loss of a lot of engineers described as excellent from
GWR▸ at St Phillip's Marsh to Hitachi at Stoke Gifford which - couple with a lot of "new" units such as 165 and 166 seems to have lead to a reduction in St Phillps Marsh being able to get though so many repairs / services at it was doing. Are we now seeing a second effect?
Where the TOC is responsible for both maintenance (turning out the units) and timekeeping / performance, it's in their financial interest to run a service even if it's on one (
HST▸ ) power car - they'll be penalised less for a late service than no service, and the penalties in both scenarios fall on them. However, with a 3rd party doing the maintenance and financially responsive for it, might it not be in the TOC's interest not to accept the unit, claim compensation from the 3rd party for not supplying to contract, and carry near-double the number of passengers in the train 30 minutes later?
I hope I'm being unduly cynical - I'm sure these things are carefully organised to avoid scenarios and temptations such as the ones I speculate about above.