When this issue occurs printing out the addresses of each item in the array shows that elements 2 and 3 have the same address, so for some reason the second element's value is being overridden by the third element's value.
If you do not move the added item's index, then its value will not be overridden once PIE begins.
Versions tested (issue occurs in all versions):
4.19.2 - CL 4033788
4.20.3 - CL 4369336
Result: During PIE, element 2 is becomes an instance of a "BP Derived Instance Object", and changing its properties also changes the properties on element 3
Expected result: Element 2 remains wholly unique from any other element
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-64327 in the post.