A fighter with a Con of +3 DOES get a total of 13 HP at first level; all characters start with the
maximum of their hit dice plus their Con mod. Given your comments about +3 to con checks, it does seem to be a bug in HP specifically.
I suggest reporting to Larian directly; they're more likely to notice and respond to bug reports submitted this way:
https://larian.com/support/baldur-s-gate-3#modal
