for me on the watch in "setting->health->health details" for me it has the latest weight that was entered by my scale which was about a week ago.
it seems to be that exact weight, and not an average of a few days
all of your weights show up in the health app on the phone?
have you tried rebooting your watch?
have you tried un-pairing and re-pairing your watch? (you should try restoring from backup, so you shouldn't loose anything.)