Dear MZina.1,
Indeed, as a general rule, ST does not disclose details about the bootloader.
To confirm that the bootloader is defective, please try a FW download using the “Boot Tool” (st8700_boot_tool_r0_8.exe) which can be found in the zip file “spi_flash_tools.zip” among the Application Notes list (saved on your PC when you have installed the GUI).
The sequence can be launched from a PC Command Prompt, as explained in the AN you mentioned (““ST87MXX_FW_Update_Application_Note.pdf”).
I put in attached a picture of my own result (I press and I maintain the “Reset” button before launching the .exe, then I release it when I see “please reset ST8700”).
Please share snapshots of your results.
PS: Note that for security reason, the chip prevents decommissioned versions : You can only install the same version or a higher one.
Best regards,
In order to give better visibility on the answered topics, please click on 'Best answer' on the reply which solved your issue or answered your question.