I've seen the same when I did scalp washes in the past (when my hair was still long). Nothing to really worry about - it's just the hairs that were about to shed that come lose from your fingers moving the hair about as you work shampoo into the scalp. I personally found that using a detangler on the hair to at least BSL (more often down towards waist) helped with combing out and getting them out after a scalp wash.
My conclusion after a while of observing shedding both for scalp and full washes was that you actually shed just as many hairs when you do a full wash - just that they are more easily rinsed out of the hair and fall down on the floor of the tub/shower and don't stay in the hair like they do with a scalp wash and get combed out afterwards.
If your hair likes it and can take it I personally found a good BBB brushing before full or scalp washes helped get many of these hairs out beforehand so you have less of them to comb out when detangling.
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