Normal behaviour.
You will notice that unformatted dates are actually numerical values with a decimal fraction. The integer portion of the number is the Day, and the decimal fraction is the Time. When a group of files (such as an Album) is selected and the value of a field for each file does not match exactly, MC will show "Varies". So when files have a date which is the same Day, but a slightly different Time, MC will show "Varies".
I can understand what your special fields do. It looks like you are supposed to be entering a string to represent a date into the field [Date Imported (Editable)], but you aren't doing that in the first image you posted, and I can't see the full field names in the second but it looks like that field is empty. So your expressions are defaulting to using the [Date Imported] field, which includes the Time, and hence you are seeing "Varies".
What you could do is modify your expression for the [Date Added (raw sortable)] field, by trimming off the decimal value your [Date Imported] field, then format it as a Date again. The new expression for the [Date Added (raw sortable)] field would be;
FirstNotEmpty( ConvertDate([Date Imported (Editable)]), FormatDate(math(int([Date Imported,0])), Date) )
Or, if I was using this as a sorting field, I would want to see the date in the format YEAR-MONTH-DAY, so;
FirstNotEmpty( ConvertDate([Date Imported (Editable)]), FormatDate(math(int([Date Imported,0])), yyyy-MM-dd) )
As above, I suspect what changed recently was that you stopped entering values into the [Date Added (raw sortable)] field, which meant that your expressions defaulted to using the [Date Imported] field. If you started entering those values again, then all would work again, but the "Varies" problem would occur if you missed entering the data.
Also, as the [Date Added (raw sortable)] field can be formatted and still sort correctly, I think the overall solution could be simplified into two fields.
Also, as the [Date Imported (Editable)] is already set as a Date field, I don't know why the ConvertDate() function is included in the above expression. But I'm not going to create your solution just to check that out.
Maybe if there was a thread that discussed this solution originally, I may take a look. Do you have a link to such a thread? But using the above you should be able to figure out what you want to do.