Same issue here, but with a ‘time-picker’ field.
I tried changing the field to ‘text’ and I had the same issue – it would not store data. All other fields work OK.
I’m on Custom Fields version 5.8.9
The field that is not saving was the first field I created.
I can write data direct into the database and see it in my post. But if I edit the data and update the post, the data it disappears.