There is a difference between left wing and someone actively involved in Labour.
Lots of people are left wing, 485k are Labour Party as of 2019. A vanishing small number of these are active activists who campaign for a party
who has campaigned online for Sadiq Khan
You dont think that is "maybe" relevant context.
Or say a "benefits claimant" being interviewed on the benefits system gives it a glowing review of how well its treated him and it turns out they're a Tory party member, who leaflets for their local MP. You dont think that just maybe an audience might find that relevant context for the statement made.
And if it was just one time nobody would probably care but the BBC has something of a penchant for finding "a refugee", "a nurse" or "a teacher" to interview who later turns out to an open and active activists within Labour.
Like this Nurse who turned out to be labour
Or last year when they could have picked basically ANY nurse Im sure to criticise the government PPE issues for Panorama but just happened to find a Labour activist.
BBC defends Panorama show that used Labour activists to criticise lack of PPE (telegraph.co.uk)
These are two I found in less than 30 seconds. It seems to happen a LOT. If they cant find someone from these groups who are willing to criticise that isnt a labour party activist, which is most of them one must question just how wide spread that criticism is and whether or not they should be giving air time to something. You might as well have ad the shadow health sec on.