Talk

No it doesn't.

Loads of babies who are now adults were mixed fed and loads more babies are being mixed fed.

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.

Of course it doesn’t. It’s formula, made to meet the nutritional needs of babies, lots thrive on it, one bottle is a non-issue.

Not that I know of but anecdotally it gave my baby an upset tummy because her body wasn't used to it and it wasn't gradually introduced (had to give formula in an emergency)

How could it possibly "undo" the benefits?

It's not got all the biological molecules of breastmilk but neither is it poison. Lots of people mix feed or supplement their babies.

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.