After nearly two years of watching my kid's phone blow through all the present screen time limits (downtime, app limits, all of them!) and then seeing the dreaded "X failed passcode attempts" box every few days and having to reset it, only to repeat every few days, I have finally figured two possibilities that have corrected this bug once I changed them:
1) I had also had my wife set as a parent/guardian on the 'family' setting, so that she would get the screen time requests. I changed this to 'Adult' instead of 'parent/guardian' for her. Now I am the only 'parent/guardian' in the 'family'.
2) I logged out of my icloud account on my mac since I noticed often times my kids screen time fail would occur after using my mac. I am now only logged in to my icloud on my iphone. This offsets many of the features of the mac, including imessage, but I'm willing to let those go if I don't have to deal with the frustration of not having screen time work as its supposed to.
I'm not sure which of these (or maybe the combination) was causing this screen time issue, but after making these two changes, it has now been over a week without the screen time failing once, the longest in a couple years for me.