Bootloop/Black screen of death after flashing boot.img - Android Q&A, Help & Troubleshooting

Well, it's a bit of a long story, so I'll try to be as short as possible.
Last year, and after hours of searching on the internet, I found the solution for "Custom binary blocked by frp lock" on this forum (here and here). Both methods promised to fix the lock without erasing my data, but I decided to try the second one (it consists of reinstalling only the boot image with Odin).
Fortunately it worked for me on the first try, and since this method was not well known on the internet, I decided to make a video tutorial to help those who had the same problem and wanted to keep their data.
The fact is that, unfortunately, this method didn't work for everyone, and in many cases it generated bootloops or black screens of death.
And my question is: does anyone know what could be the cause of this?
I tested it on SM-J710MN and SM-G532M, both running Android Marshmallow, and the bootloops/black screens occurred on devices running Android 6, 7, 8 and (I guess) 9 and 10.

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Starting with Android 6 FRP on any such Android device is automatically enabled once a Google account is logged in to a device. The only way to get rid of it is by removing the account from the device into which it has been synced.

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jwoegerbauer said:
Starting with Android 6 FRP on any such Android device is automatically enabled once a Google account is logged in to a device. The only way to get rid of it is by removing the account from the device into which it has been synced.
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Thanks for answering.
Maybe I did not express myself in the correct way, but actually I was referring to the block that does not even allow to boot the device and only shows words in red, as seen in this image.
{
"lightbox_close": "Close",
"lightbox_next": "Next",
"lightbox_previous": "Previous",
"lightbox_error": "The requested content cannot be loaded. Please try again later.",
"lightbox_start_slideshow": "Start slideshow",
"lightbox_stop_slideshow": "Stop slideshow",
"lightbox_full_screen": "Full screen",
"lightbox_thumbnails": "Thumbnails",
"lightbox_download": "Download",
"lightbox_share": "Share",
"lightbox_zoom": "Zoom",
"lightbox_new_window": "New window",
"lightbox_toggle_sidebar": "Toggle sidebar"
}
But beyond that my question is about why does the bootloop or black screen happen after flashing the boot.img file? Does it have to do with a different/incompatible version of Android/binary/csc or something like that?