Commented Unassigned: "Number saved as text" [15186]

When i open files XLSX edited by EPPlus in Microsoft Office Excel i get a warning (notice, comment) message for EACH cell with custom formatted number. This is really not cool.

1. Why this happens?
2. How to fix it.
3. If it is not possible to fix, WHY?

Thank you.
Comments: "Number saves as text. Convert into number?"