1) Create a template with host prototype and set its inventory mode to 'Manual'
2) Create autoregistration rule that adds a host and links this new template
3) Start a server and agent - new host was created and a template was linked, but the host prototype on a host has inventory mode set to Disabled
4) Unlink and clear template from the host
5) Link template manually - host prototype was linked again but this time the inventory mode was copied and is now set to 'Manual'

Possibly linked with https://support.zabbix.com/browse/ZBX-10587
where front was forced to link a host prototype inventory mode, but the fix was not propagated to server.