ClickForFreeBeer
Recruit
Custom battles definitely use realistic. Are you on the latest versions? Because I seem to remember it being like that but I feel like the last few times of been playing it's actually been reasonable and I'm rarely getting hit unless I get within 100m and run in a straight line.It may also be somewhat dependent on difficulty settings, with things below "Bannerlord" (what setting do Custom Battles use?) not having these sorts of sniper shots happen as often.
Just look up archery against moving targets, an experienced archer can lead shots pretty well even against fast targets, and considering bows were also frequently used to hunt, theoretically they'd be used to hitting a running target at similar speeds to a horse.