MKR1000 & Maximum Flash storage

I can't find what is the maximum internal flash storage I can use.
I have notice that after writing 1240 bytes, my MKR freeze and I need to perform a factory reset.
Does anyone have some information about this limitation (for a chip that provides 256kB, it sounds quite low to allow only 1240 bytes of persistent data),
Thks

Additional investigation showed that in fact the limit is 1 kByte but no idea on how to change this limitation