From the information provided, I am a little unclear. Were you contacted and asked to come out specifically to clean the water bucket because the barn owner was indicating that they would not otherwise? If that was the case, I would absolutely take issue with that.
If I am paying for, basic care that includes ensuring my horse has access to clean water. My horses access to clean water should not be dependent on my ability to come out on a specific evening and clean a water bucket.
If however, it was more of a eye roll gosh horses are gross lol kind of moment, but they would’ve cleaned it no matter what and you came out before or offered to do so, I don’t think that is weird at all.
If stalls are done in the evening, along with water buckets, I would not necessarily expect someone to break that routine to dump a water bucket early. Personally, I would because it would just gross me out, but realistically the germs and funk are there whether the book bucket soaks for 20 minutes or 12 hours. I would definitely expect it to be cleaned before being refilled.
If a horse was a chronic bucket pooper then I think that is where a barn, owner and horse owner collaborate around creative bucket, hanging, or other ways to try to mitigate the problem.