Sounds fishy, possibly fraudulent. Who are these invoices payable to? If they’re to the strata manager, even more suss. Whilst anyone can claim accounts rendered up to six years ago, it doesn’t make any financial sense to let these matters go for months or years. The manager needs to account to the plan by providing basic records. Have you had your plan audited by an independent auditor? If so why didn’t they pick up on the uncharged amounts, worth asking them.
If the invoices are for strata services they need to be verified against the charges scheduled in the Management Agreement.