Project Resource Assignment Custom Fields do not maintain the selected value

We have a number of custom fields in the "Rates and Additional Information" section of the "Edit Project Resource Assignment" page.

These have been working as expected and we can still select the values for each of the custom fields created.

These fields have been used for over a year without issue and we've not deployed any additional config recently.

Now, when a value is selected from theses fields which is not the default value, we can save the change. However, when you go back into the record, the value has returned to the default value or on occasion a value which hasn't been selected.