I got the same issue;
used following workaround:
temporarily disabling PostTypesOrder, selecting the page in acf, saving and reactivating PTO;
if you take a look at the custom field again the selected page makes no sense, still works though;
after reading some feedback in the PTO-forum I used the following fix which is quite acceptable for the moment:
Go to Settings->Post Types Order and uncheck the “Administrator Sortierung”, guess it’s named something like “administrator sorting” in english backends;
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