When you breeding horses, if one (or both) of them are in the minecart, they won't try to produce a foal, even they are close enough and tamed.
If you break the minecart after they are fed, they will produce a foal.
It is worth noting that, if you try to breed cows in the minecart, they can produce a baby cow.
Thank you for your report!
We're tracking this issue in MC-60253, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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