I would encap that with hydrox which is acidic and also has peroxide, but you can mix it up stronger say 4 to 1
maybe use a bonnet rather than a pad and leave to dwell before extracting
I had a similar problem this time last year, at a specsavers store in west yorkshire, carpet tiles were badly worn and there was evidence of bitchemus wick back, I used hydrox which eradicated the problem
Stuart