So I find it will go to 50% if the charge limit is changed in the IO app. However if adjusted in the Tesla app, its fine.I've seen exactly the same thing, it's pretty annoying! Because I WFH 3 days a week, on the WFH days I'm happy for the car to be ready by 11am (since, if it offers a 9am-11am charge slot, I can make use of that while at home), but on WFO days I need it ready by 8am.
Changing the ready time seems to set the vehicle charge limit to 50%. I need 50% for a round trip to the office, so I really do need to be careful to re-adjust the charge limit to a more sensible, less squeaky-bum amount.