What my comment does is telling You the cause of the issue and for anyone coming this way - it is the correct answer to mitigate the issue - should it occur again.
I don’t know what You think you did - but it was sheer luck You got it working and what you did had nothing to do with the cause.
You don’t have to jump through hoops to get there - before launching the installer - open a terminal and run
sudo sgdisk --zap-all /dev/sdx
This will effectively remove all remnants - superblocks, signatures etc.
The reason this issue even exist is because Plasma mounts the partition automagically On Attach - and removing the settings described prevents this.
MBR is a special part of the disk used to hold the bootloader - to remove it You will have to create new GPT partition table.
Any recent system can use BIOS/GPT but requires a special BIOS BOOT partition 1M unformatted (0xef02) - this partition is then used to hold the grub loader.