From past few days am observing a different agent named DT_server(I have never observed previously) in agents overview dashboard. PFA fro details and tell me why it is coming and that too in offline status.
DT_Server and DT_Server_Frontend are the two agents instrumenting the Dynatrace server processes for Self-Monitoring.
Regarding your incidents, "Ambiguous Agent Mapping" means that the agent cannot find a System Profile to map to (based on the mapping rule). The reason I suspect it's not mapping is that the rule for the Self-Monitoring profile is that agent name equals "DT_Server" and instead you ended up with "DT_Server" so the rule doesn't apply.