When performing a 'Group by' on a list column/field, the records within each of the group are not correctly grouped. They can appear in an "empty" named group or even other groups.
This is caused by the column/field being configured using a calculated value set on the dictionary entry of that field.
A calculated field works to show the combination of several fields in one field, which only displays on display of either the form or list.
- Open up the records not correctly grouped
- Save the record, capturing the calculation done by the calculated value
- Open up the list and validate