If stirring negatively affects crystal purity, why is it so?
&quot;Strong stirring&quot; is a rather imprecise term.  There is such a thing as too much speed.  Rule of thumb for APIs, if you're working with a magnetic stirrer in a round bottomed flask in a lab (rather than in some industrial vessel or pilot plant setup) 350 rpm is too fast (and no, I can't quote a reference on that, it's one of those things you get told and then confirm by experiece - usually by being a wise-arse and bucking the &quot;established wisdom&quot; and ending up with a product that doesn't make medical grade).
Agitation for different crystallisation processes are generally kept in certain narrow bands to avoid inclusions as wel as a number of other reasons.
An example is described in Chow et.al [1] where when crystallizing paracetamol from water in a 500 mL round-bottom flask with p-acetoxyacetanilide present, increasing the stirring speed from 200 to 320 rpm increased product purity, but increasing further to 400 rpm lead to decline in product purity.
According to Meedara et.al. [2] stirrer-particle and interparticle collisions can facilitate crystal attrition  in suspensions with high energy agitation, leading to attrition-induced inclusions.
Urwin et.al. [3] did a reasonably thorough job testing various methods and reasons for presence of impurities in crystals, and is worth a read.
Another good reference is The Handbook of Continuous Crystallization, start with chapter 1 [4]
That said, in crystallisation especially of APIs, mechanisms can be compex and are still not completely understood. All kinds of workarounds have been developed, such as e.g. ultrasonics to help prevent agglomeration when crystallizing paracetamol from certain solutions.
If you have no literature on the process you are attempting, finding the &quot;sweet spot&quot; where you get the greatest purity of product, is only going to happen with a lot of experimentation.
[1] Chow, A. H. L.; Chow, P. K. K.; Zhongshan, W.; Grant, D. J. W. Modification of Acetaminophen Crystals: Influence of Growth in Aqueous Solutions Containing P-Acetoxyacetanilide on Crystal Properties Int. J. Pharm. 1985, 24 (2–3), 239– 258,  DOI: 10.1016/0378-5173(85)90024-9
[2] Meadhra, R.; Kramer, H. J. M.; Van Rosmalen, G. M. Model for Secondary Nucleation in a Suspension Crystallizer AIChE J. 1996, 42 (4), 973– 982,  DOI: 10.1002/aic.690420410
[3] Urwin, SJ, Levilain, G, Marziano, I, Merritt, JM, Houson, I and Ter Horst, JP
A Structured Approach To Cope with Impurities during Industrial Crystallization Development Organic Process Research &amp; Development 2020 24 (8), 1443-1456,  DOI: 10.1021/acs.oprd.0c00166
[4] McGinty, J, Yazdanpanah, N, Price, C, ter Horst, JH and Sefcik, J  Nucleation and Crystal Growth in Continuous Crystallization , in The Handbook of Continuous Crystallization, 2020, pp. 1-50 DOI: 10.1039/9781788013581-00001