When using /fill command rotated blocks such as Stair blocks / logs / pillars etc. loss their orientation then changed to another block type.
Example! A east facing stone stair block will default to facing south when changed to a stone brick stair while using the /fill command to change the stair block type.
To reproduce: Place stone stairs in multiple directions then use /full command to change them to stone brick stairs or any other type of stairs. All will be replaced but all will be facing south.
This is very arguably intentional design. The
facing
nbt data can be specifically clarified in the command: