If you are receiving advanced premium tax credits, the insurer is required to pay your claims during the first 30 days of the grace period. After that, during the second and third month of the grace period, the insurer is allowed to hold your claims and only pay them if and when you get caught up in your premium payments before the end of your 90-day grace period.
Also note that if you are seeking new coverage from an insurer to whom you owe premiums for prior 12-month coverage, the insurer can request that you pay your past due premiums as a condition of enrolling you into new coverage. This only applies to insurers to whom you owe premiums for prior 12-month coverage (i.e. if you switch to a new insurer, the company cannot condition enrollment on paying premiums you owe to your previous insurer). Insurers can impose this obligation during both open and special enrollment periods. (45 C.F.R. § 156.270(d); 45 C.F.R. § 147.104; 82 Fed. Reg. 18346, April 18, 2017).