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If it were me, I would grab a cheap boar brush from target, a different soap, lather in a bowl, then give it the same amount of time. If this changes things, then rotate in "1" item at a time and...

Could it be something in the sink, since you are using it to soak your brush?
I soap my brush in a cup, only use it for the brush.
Designate a cup or bowl just for your brush & lathering for a...

Are you using a pre-shave routine? What is it?
There is a common denominator somewhere it sounds like.
Tell us more about your shave routine.