Hi Julie,
Is this multiple choice field part of a relationship? Is it possibly the proxy field for a relationship? If so it is possible that you have both the reference field and proxy field of the relationship on the form at the same time, when this happens Quick Base isn't sure which of the two to trust so it blanks the entry out. You can find this out by right clicking on the field name and opening up the field properties, from there you can see if it is in a relationship and if so if it is the proxy. That can then help you track backwards to see if both fields are on the form. That tends to be the most common reason a selection in a multilple choice doesn't stay in place after save.
