I found this bug by reading the code in lifetime.d, not by a test case. Struct postblits are not called when an array is copied due to an append, and they should be, since there could still be references to the original array.
https://github.com/D-Programming-Language/druntime/pull/29
Fixed https://github.com/D-Programming-Language/druntime/commit/97d6b19e37b73651e9fc73fe80f82238183397f3