Modifying an entity's nbt data will reset its rotation (hitbox & model).
It will reset to the last rotation which was set by a command (/tp or /data) and in case it was never teleported/rotated it will face south.
MC-183962
Entity rotation resets when modifying nbt
Resolved
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Please check if that's still an issue for you in the latest 1.16.4 stable release or the latest 1.17 development snapshot.
If this is still an issue, please provide proper reproduction steps or a small video with the debug screen enabled (hit the F3 key) demonstrating the issue.