The Modern Edit form appears to have some issues with the Combo Boxes linked to SPO Choice Fields.
Set(varThisListItem, Defaults(SPOListName));
NewForm(Form1);
If I click the +, I get a new form for the list. I enter the Title into my Project Title field which is a Single Line of Text in my SPO list and select one of the options in the Combo Box for the "Project Source", then save the record (a Save icon that is simply "SubmitForm(Form1)"
In this screenshot, the Project Source was set.
Adding another record is where the problems start.
Click on the + again, get a new form. Enter a new Title into the Project Title field. On this new record, I do not select a value for the Project Source (the SPO list does not have a default set for this column). Save the record.
In this screenshot for Project Title 2, the Project Source was not set by me, it was set to the same value as the previously entered record. Again, the SPO list does not have a Default set:
Look at the list to see what's been saved.
The first record has the correct title and Project Source selected. The second record has the correct title in there but the Project Source is the one from the first record entered.
If I keep entering records but never select a Project Source, it just keeps saving with the previously entered source. If I add a record but give it a new Project Source, it saves the correct Project Source on that record. Subsequent records added that do not have a Project Source selected get the previously selected Project Source.
Other Choice Columns that have defaults defined do the same thing. If the combobox for that field isn't set, instead of being set to the column's default, it gets set to the previously added record's value.
Within the same app I added a Classic Form that is using the same data & code and that form is working properly.
I hope that all makes sense. Authoring version is 3.24074.14