How to reproduce:

1. change inventory mode to automatic from frontend or API
2. execute this API call

{"jsonrpc":"2.0","method":"host.update","params":{"hostid":"10084","inventory":{"type":"1"}},"auth":"7ab185693c686165752a33df2afe143b","id":1}

3. back to frontend, inventory mode is changed to manual

I think updating inventory object should not touch inventory mode setting.