Reproduced 3/3 times, issue does not occur in //UE5/Release-5.0 - CL 20979098 Binary. Confirmed Regression.
Only seems to occur with User Defined Structs. Testing with other variable types, but all of them kept the toggle visible after migrating.
If you enabled the Expose On Spawn setting before migration to the blank project and try to spawn MyActor using the Spawn Actor from Class node the MyStruct pin will appear. If you add an additional MyStruct variable to MyActor after migrating, the toggle will still be missing.
Actual Result: The toggle for "Expose on Spawn" is missing.
Expected Result: The toggle for "Expose on Spawn" is still there.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-172915 in the post.