I have starting chunks set to 9 and get different amounts of starting chunks each time I generate a world. There is nothing in the log that indicates an error but I can use the same seed and get 5 chunks or 2 chunks or 1 chunk. I have no idea whats going on.

Ok this was probably some kind of mod conflict with a bad structure causing errored world gen.

Ok - I take this back, I started running into this again, just less frequently. I think it gets stuck, fails to generate chunks and then never recovers because the .dat file for the mod is busted/stuck. I wish there was something in the log to try to troubleshoot this.

The issue is most noticable when adding new chunks or with additional starting chunks, and occurs more frequently when using the sealed world option.