After some time, flash is corrupted. At least - the lower two bytes are corrupted (the reset vector).
Flash was not written at the time of the problem.
The problem is quite common in hardware. But it is not possible to reproduce it on the stand.
1) do you have a pullup on !RST ?
2) do you have decoupling no more than 5mm from Vdd?
I would like to echo what luweiguo said, the schematic will be a big help for diagnosing this issue. Also you may want to take a look at our KBA regarding our guidelines to avoid flash corruption: https://www.silabs.com/community/mcu/8-bit/knowledge-base.entry.html/2004/02/14/flash_corruption-u4VR
I did not see issue in your schematic that could explain the flash corruption issue.
1. usually silabs don't recommend to add capacitor on RSTn. only a pull-up resistor is recommended.
2. it looks you don't use USB function that was available on this device.
3. you have lot of external component for C2/program interface. usually only 3 pins are ok to program/debug the F321, GND, C2D,C2CK and you don't need other external component (except item 1) for these pin.
4. you may need to pay special attention on the LDO and 12V supply. you could probe the VDD of F321 to see if it violate the spec.
As far as i know the external capacitor on RSTn may limit the C2 speed.
As far as i know the external capacitor on RSTn may limit the C2 speed
GET RID OF IT
in the Cygnal days some .... 'solved' noise by putting a cap on !RST and that morphed into an urban legend that the cap was beneficiial BULL....