From the docs, you cannot set multiple log levels in the SERVICE section.
If you haven't already, you should be setting up a Filter section to match the specific logs you are looking for, then overriding the Log_Level in that section. Filter docs say:
Log_Level: Set the plugin's logging verbosity level. Allowed values are: off, error, warn, info, debug and trace. Defaults to the SERVICE section's Log_Level.
Since SERVICE log_level is accumulative, e.g.
Values are accumulative, e.g: if 'debug' is set, it will include error, warning, info and debug.
And Filter defaults to the SERVICE log_level, you will get all logs of all log levels.
See the Service and Filter sections in the docs: https://docs.fluentbit.io/manual/administration/configuring-fluent-bit/classic-mode/configuration-file