explain_all_plans_execution_stats.js has been observed to fail in Evergreen nightly runs with error "Index not allowed on, or a prefix of, the encrypted field a", but this test does not use encryption. The problem is that the test uses a collection named "test" but does not drop it first, and it uses generic field names "a" and "b" which are also common. Thus if a prior test also used a collection named "test" with an encrypted field "a", this test will fail in this manner when it tries to create an index on field "a".
 

The fix is to drop collection "test" at the start of this test.