A netherfortress that was loaded pre-20w21a does not spawn any mobs when it is being loaded up in 20w21a. The biome this netherfortress is located in is soul sand valley.
Environment
20w21a, Windows 10, Java version 8 update 251 build 1.8.0_251-b08, NVIDIA GeForce GT640M, Intel i5-3210M CPU @ 2.5GHz.
Duplicate of now reopened MC-182743.