For the daemon configuration option 'SourceIP' the online documentation as well as the distributed configuration files state:
Source IP address for outgoing connections
Well, I'd argue that database connections made by the daemons could be considered as outgoing connections too
Apparently this does not apply for such connections made by daemons.
Either this should pointed out more clearly in the documentation or fixed to comply with the documentation.
Personally I'd very much welcome the latter way, since I'm thinking to bind Zabbix server to a dedicated service IP - thus marked as a bug