NAND flash devices as assembled in SD Cards and on some Zbrain devices have the properties not to hold the information for eternity.
The charges, that represent the informations will degrade under certain conditions. There are big differences between different cards.
If the memory is never read, it will take some years until the information gets unreadable.
With each reading the neighboring cells of the read cell discharges a little bit. This process is called read disturbance.
After about 100'000 reads of the same file, the first bits can get lost. This is compensated by the ECC mechanism. If too many bits have changed their value, the file becomes unreadable. For the on board NAND flash the Zbrain software has built in algorithms to minimize this effect. For the SD Cards, this is not possible. Storage devices with higher density (this mostly goes hand in hand with higher capacity) are more vulnerable than others.
To prevent your system from loosing data, try to minimize the reads on each file.
For bitmaps you can use the constant bitmap work around