Hi
I have this issue with the date&time field in modern SharePoint sites (all tenants where I've tested it). Date field only works perfectly, but adding the time to the field causes it to not save the data at all. We're using the date field in Communication site Site Pages libraries, to filter posts for publishing time. It would be crucial to get the time to the publishing date&time, but currently have needed to go with the date only.
See pic:
Reading this forum and other resources in the Internet, seems that this has been a known issue for way over a year already and still is not fixed!