Checkbox set clears it's own field in WebDirect

I Have a text field Staff::Availability that I use to record staff members' availability to work morning and afternoon shifts.
Display/data entry is via a checkbox set.

For presentation and ease of use, I have placed two copies of the same field side by side, one using a value list for AM shifts, and the other using a value list for PM shifts.
(I arrived at this point because a single value list was too difficult to present in a user friendly way across three OS's and Pro, Go and WebDirect)

It works fine everywhere except WebDirect. When accessing with WebDirect I am able to enter values using the checkbox set as expected in either the AM or PM side, but as soon as I switch from one to the other, the first selection I make on the other side replaces any existing contents of the field rather than adding a return delimited item to the list. Continuing to check off items in that side works as expected. Committing the data by clicking/tapping outside the field retains the data. Reentering the last side entered retains the existing data and allows the user to continue adding or removing items from the checkbox set. Making a selection from the other version of the field immediately replaces any existing field data.

The field is in a FMS hosted file being accessed via a separation model file, in case that matters.