TOI-849b is a rocky planet 40 earth masses in size (https://www.nationalgeographic.com/science/article/bizarre-planet-largest-known-rocky-world-40-times-as-massive-as-earth)

Kepler 138d is a mini Neptune gas giant 2.1 times heavier than the earth meaning TOI-849b is 19 times larger than it.

Charon and Pluto have a 2:1 mass ratio meaning a 19:1 ratio can definitely be stable and is not really a double planet system.

Also how this can be formed TOI-849b is believed to be the core of a massive gas giant which somehow lost its gaseous layers (there are inconclusive theries on how), so maybe it could have had the moon from when it was larger or maybe the moon was a Rogue planet that was captured in orbit.

The likelihood of this configuration is very low but I would say it is possible and the universe is huge so maybe it could happen somewhere.