Summon a boat with a rotation of [1.4f,0f], leave and re-join the game, wait 1200 ticks, and focus on the boat's rotation. You will see the boat rotate on its own.

/summon oak_boat ~ ~ ~ {Rotation:[1.4f,0f]}
/tick sprint 1200

(See the attached video.)

This issue can be observed not only with boats, but also with the following entities.

• minecarts
• display entities
• interaction (with F3+B)
• item (with F3+B)
• armor_stand (with F3+Shift+I)
• etc. (possibly all entities?)

The exact condition this issue occurs is when a certain amount of time has passed (different entities require different amounts of time), or the OnGround value has changed. Entities usually pass their rotation values to the client side in 360/256 increments (to fit in a byte), but when the condition is met, the rotation values with exact precision are sent to the client, causing the entity's rotation on the client side to change.

For me, the impact of this issue is most severe on displays, for example, imagine you're entering a world decorated with many display entities. Exactly 400 ticks after joining, all of a sudden, all of the display entities will start rotating at the same time, which is never the behavior you want as a mapmaker.