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Give it some more time,Simpson,Rooney,Thater,Plisson, all make fine top of the line brushes,but all will shed to a point.
All my brushes are homebrew TGNs,they all shed in the beginning,than they...

How many times have you used the new brush?
some shedding is normal when you consider a 22mm knot has an estamated 10K hairs.
After maybe 20 use cycles this should subside but a hair or two will...

Since you bought it used I think your only recourse may be to re-knot it:(