There is no general answer, with reference to the fireman's rule. See Minnich v. Med-waste for a review of the various different answers given by courts, and numerous citations. In some states, there is a rule that limits liability to emergency personnel who are damaged by a third person's negligence. One version posits a very limited duty only to "refrain from willful or affirmative acts which are injurious", and it is reasoned that officers are aware of the risks inherent in their profession and have assumed those risks. There are policy arguments in support of the rule, including the fact that the injuries are compensable through workers' compensation thus liability for on-the-job injuries is properly borne by the public (these are rationales advanced by various courts in their rulings). "Line-of-duty" injuries are particularly insulated from findings of liability.
Some states have abolished the rule, for example Virginia allows liability when a person knows of responder presence, or where negligent act or omission violates a statute, or the injury was independent of the officer's duty being carried out. The SC Supreme court summarizes that
those jurisdictions which have adopted the firefighter's rule offer no
uniform justification therefor, nor do they agree on a consistent
application of the rule. The legislatures in many jurisdictions which
adhere to the rule have found it necessary to modify or abolish the
rule. The rule is riddled with exceptions, and criticism of the rule
abounds
The present question focuses on a specific subset of circumstances: police officers (not all emergency responders), and injury arising from executing a search warrant. Although initially the fireman's rule was a rule about firemen, it quickly extended to police. No ruling has suggested that police officers and firemen should be treated differently, and no ruling has suggested that there are different liability standards for the execution of a search warrant, versus any other law enforcement action.
See Moody v. Delta Western, Inc., 38 P.3d 1139 and Waggoner v. Troutman Oil for compendia of holdings on this rule. Counting cases, the rule seems to generally hold, but the specifics of the rule are not uniform across states. As pointed out in Moody,
the Firefighter's Rule does not apply to negligent conduct occurring
after the police officer or firefighter arrives at the scene or to
misconduct other than that which necessitates the officer's presence.
Such misconduct may include failure to warn of pre-existing known but
hidden dangers
(which is exactly the case that you describe) as established by the courts in Maryland and California. Whether or not an exception to the rule has been made explicit in a given state, the rationale of Flowers v. Rock Creek Terrace, Neighbarger v. Irwin Industries and related cases could be persuasive .
