The provision could delay their rate increases in the short time, but undermine efforts to keep premiums affordable for everyone, the Los Angeles Times reported Tuesday.
The loophole enables insurers to extend existing policies for almost all of 2014. Most provisions of the law go into effect Jan. 1.
Arkansas officials are encouraging health insurers to take advantage of the exception by resetting customers' renewal dates for the end of December.
Policy experts fear insurers could use the loophole to renew only younger, healthier policyholders. That would leave an older population in the new government insurance exchanges, driving up costs and premiums.
The provision could affect some of the 15 million Americans who purchase their own coverage and millions more uninsured expected to join the exchanges. It would not affect 150 million in the country whose employers provide their health benefits.
Changes in the federal law could cause premiums to rise an average of 30 percent for 1.3 million existing policyholders, California officials have estimated. Legislators there are working on proposals to address the uneven way in which policyholders could be treated because of the loophole.