stm32 - How to overwrite flash memory on STM32L series

I am trying to write a known pattern (ie 0xFFFFFFFF or 0x00000000) on top of already written flash memory, to invalidate portions of it for a primitive file system. But it doesn't work for me on the STM32L series as it does on the STM32F series.

I am used to the STM32F family of microcontrollers, where the flash memory is erased to 0xFFFFFFFF and written with 0's. You can write anything you want to erase memory, ie

 write 0x00001234 on top of 0xFFFFFFFF -> 0x00001234

and you can write 0x00000000 (all zeros) on top of anything

 write 0x00000000 on top of 0x00001234 -> 0x0000000

I am now using the STM32L family (low power), and the flash memory is totally different. It is erased to 0x00000000, and written with 1's. However, I don't know how to reliably write all ones. For example, if I erase, I can do this

 write 0x01020304 on top of 0x00000000 -> 0x01020304

but if I try

 write 0xFFFFFFFF on top of 0x01020304 -> 0xFFFFFFBF !!!

Note that the final answer has a B in it. It is not all ones. In fact, if I write bytes 0x00 to 0xFF to a freshly erased page of memory, and then write 0xFFFFFFFFFF all over it, I get very glitchy results:

ff ff ff bf ff ff ff ff ff ff ff ff ff ff ff fb
f7 ff ff ff fd ff ff ff ff ff ff f7 ff ff ff ff
fe ff ff ff ff ff ff ff ff ff ff 7f f7 ff ff ff
ff ff ff fb ff ff ff ef ff ff ff ff ff ff ff df
fe ff ff ff ff ff ff ff ff ff ff 7f f7 ff ff ff
ff ff ff fb ff ff ff ef ff ff ff ff ff ff ff ff
ff ff ff bf ff ff ff ff ff ff ff ff ff ff ff fb

f7 ff ff ff fd ff ff ff ff ff ff f7 ff ff ff df
f7 ff ff ff fd ff ff ff ff ff ff f7 fe ff ff ff
ff ff ff bf ff ff ff ff ff ff ff ff fd ff ff ff
ff ff ff fb ff ff ff ef ff ff ff ff ff ff ff bf
fe ff ff ff ff ff ff ff ff ff ff 7f fb ff ff ff
ff ff ff fb ff ff ff ef ff ff ff ff ff ff ff bf
fe ff ff ff ff ff ff ff ff ff ff 7f ff ff ff ef
f7 ff ff ff fd ff ff ff ff ff ff f7 fe ff ff ff
ff ff ff bf ff ff ff ff ff ff ff ff fb ff ff ff

Here is the pseudo code I am using (FlashWrite is a wrapper around the STM std periph library). I tried writing a pattern of 8 writes with the bits shifted <<1 each time, and that actually gave me what I wanted (all ones) but I am not sure this is reliable.

 uint32_t pattern = 0x04030201;
 FlashErasePage(0x0801E000,FLASH_PASSWORD);
 for(int j=0;j<64;j++) {
    FlashWriteArray(0x0801E000 + 4*j,(uint8_t*)&pattern,4);
    pattern += 0x04040404;
 }

 for(int j=0;j<64;j++) {
#if 1

     // write once
     uint32_t pattern = 0xFFFFFFFF;
     FlashWriteArray(0x0801E000 + 4*j,(uint8_t*)&pattern,4);
#else
     // write shifting bit pattern
     uint32_t pattern = 0x01010101;
     for(int i=0;i<8;i++) {
        FlashWriteArray(0x0801E000 + 4*j,(uint8_t*)&pattern,4);
        pattern <<=1;
     }

#endif