Interesting, I didn’t release the guys had flashed different versions of the firmware across the production units. This is useful as I can now see the specific fix made. They do not relate to how the SPIFFs works but rather how they publish large packets. From your comments above, I suspect you have a mix of both firmware.
Version 2.1.7(1) was the original firmware supporting the ESP8266 put out in August. This does not include the Button 2/3 ability to reset to AP mode nor the “packet fix”.
Version 2.1.7(2) was an updated firmware that was checked-in in December. This includes the new B2/3 selection, the “fix” and a new variable in the “http://os_ip/jc” command that provides last reset time “lupt”.
You should be able to confirm the firmware version of each unit via the Web App -> About.
If the one experiencing problems is the older 2.1.7(1) then I would swap it out for one of the newer firmware units and that might solve the problem for your main OS unit. You could still potentially experience the problem with the older firmware units once their log records grow but you may have bought yourself some time.
I would then get on to OpenSprinkler support for assistance in upgrading the other units or explore how to build the firmware directly from Github (not easy unless you are pretty technical).