Lots of dodgy myths circulate on this. Absolutely no decent studies on the issue afaik looking at effects small amounts supplementation bf baby. Plausibility wise, highly unlikely to have negative effects. Most it might do, if baby still very small - is maybe reduce supply, but if you have got into the bf "established" point then no real need to worry about that. And particularly if it's just small amount supplementation.
I recall something (in young babies)- formula does produce changes in gut bacteria, but this article / study babies had a small amount of formula and back to bf the gut bacteria had returned to previous composition in a week maybe. Obviously there are no clear results evidencing these changes as harmful or evidencing any significant impact of formula feeding (except perhaps in prem babies). Sorry can’t remember more detail.