I have had the same with a JBD BMS since the beginning. I set it to 280Ah and it suddenly resets to 100Ah. I thought it was due to the iOS app I was using, switch to another - same thing happens. It super annoying, as I loose the current SOC every time it does that.
On iOS SmartBMS Utility is free and open source and can change all settings.
It is also available on the Mac App Store for Mac OS with the same interface and functionality.