Is it uncommon to develop chronic sinusitis problems after a root canal?

Yes uncommon: Sometimes the root apices of maxillary molars can intrude into the maxillary sinus cavity & an infection can cause schneiderian membrane thickening & fluid accumulation & infection within the sinus. This usually resolves after rct therapy & resolution of infection. Sometimes, residual infection can remain, especially if the rct was inadequate. Extrusion of gutta percha & sealer can irritate.

Yes, but...: Root canals don't cause chronic sinusitis. However, a chronically infected or abscessed upper molar tooth can lead to chronic sinus infection. Sometimes it's hard to tell whether the infection began in the sinus or in the tooth. You may need a ct. You may also need retreatment of the root canal or even extraction of the tooth. You should see an endodontist, and perhaps an otolaryngologist as well.

Yes: Chronic, long term problems should not be related to properly done root canal therapy. It is normal to have short term sinus issues due to the trauma of root canal manipulation, but chronic problems are not common.

No: One of the most common causes of unilateral maxillary sinusitis is a complication of a root canal operation on an upper molar tooth.

RCT/sinusitis?: Yes it is uncommon but can and does happen. Should be evaluated by both an Endodontist and ENT specialist.