Unfortunately, that is normal procedure for USCIS. It is a fact that USCIS has practiced last in, first out for some I-751 cases while others sit for years. We filed our I-751 in March 2019. It became "Ready for an interview to be scheduled" in May of 2019. We had no RFEs. Nothing happened until wife filed for citizenship in August of 2022.....40 months after filing the I-751. Both the I-751 and N-400 were approved after a combo interview in December 2022. Sadly, some cases get left behind. A WOM might help in your case.