[Solved]MySQL index work in one table and not work in another table when using IN
In both cases, you should and will get index utilized in your explain plan.  I am not sure why you are getting different plans, but I have got the same by executing your code as it is.

First plan

And the second plan:


There may some fragmentation/table stats issue.  would you execute OPTIMIZE TABLE or ANALYZE TABLE on your tables to see the correct plan?