Loading a custom dimension with very high height and low min_y (tested with height 2032 and min_y -2032, dimension type file attached) causes main worker threads to crash and die (note: the game keeps running and does not produce a crash report, however the stack traces are still written to the logs, which were attached to this report)
This could potentially be related to MC-217702, as this happens during feature placement.
Fixed in 21w14a, can no longer reproduce