Unconsistant behaviour when saving files in overlay

Hello everyone,

I am building an openwrt firmware for an old mt7620 board for commercial purpose (captive portal)

I am encountering a strange behaviour when I try to save updated config files. I am trying to figure out the reason.

If someone have an idea or have already seen such a situation, this is an example of what happens :

From the console, I go to the /overlay/upper/etc/config:

- Before reboot
     - If I modify the existing "network" file (from the firmware compilation)
     - I delete another file i.e. "dropbear" (from the firmware compilation too)
     - I create a new file for example "test"
     - I create a folder "new_folder" with a file inside

- After reboot:
     - changes in the existing "network" file are not kept
     - "dropbear" file reappeared
     - The new file for example "test" is still there
     - "new_folder" with its file inside are still there

- Finally :
     - if I delete the new files "test" and "new_folder", after the reboot they are effectively deleted

I imagine that, may be there, is an issue with the overlay folder that only "see" the new files created but not the one from the compilation.

Thank you by advance,

Best Regards