I had the same issue on a site that I have inherited developer duties for. I found that with the Classic Editor Plugin Disabled although parent pages were still missing from the dropdown – I was able to key them in to the attributes box (with the Classic Editor Plugin enabled – direct keyed entry was prevented) and they were findable and settable. Not a solution but a workaround via the wordpress dashboard rather than via mysql.
Also note pages must be published by default to show How can I set a draft page as parent without publishing?