Is system memory protected from read?

[ROlej.1]

Recently we have received STM32F103RBT6/T7 samples which could not be flashed using internal bootloader. We have checked working devices and under 0x1FFF F000 they have readable bootloader firmware code. Problematic samples have FF bytes in the same location. Are they damaged or is there any read protection for them?

[Piranha]

https://community.st.com/s/question/0D53W00001eZGxYSAW/why-could-stm32f103-flash-write-last-so-long

You didn't tell whether the chips where bought from reputable source. If no, most likely the chips are fake.