@Heru, is the schematic in the archive you sent before you opened it with 5.1 or after? If it's after, would you please attached the before schematic was opened and saved with 5.1? If this is the original schematic file, do you remember what version of KiCad was used to save the file? The only other thing I can think of is something is corrupting the thickness value during editing and it is being saved as bold. The schematic that you sent have the text and labels are indeed set to bold.
@Heru, is the schematic in the archive you sent before you opened it with 5.1 or after? If it's after, would you please attached the before schematic was opened and saved with 5.1? If this is the original schematic file, do you remember what version of KiCad was used to save the file? The only other thing I can think of is something is corrupting the thickness value during editing and it is being saved as bold. The schematic that you sent have the text and labels are indeed set to bold.