I've trivialised the necessary change. By adding an empty element to the list, resetting the toolchain between iterations has no impact on the build environment.

Before this change it seemed like the zero element was either being cleaned out or somehow being evaluated as equivalent to None.

I confirm that this fix solves our problem (#2911).

Thanks @ocaisa !

@ocaisa Is there any way in which we could enhance the tests to catch this issue (and prevent from re-introducing it)?

@boegel I guess we could check that the toolchain is reset-able...I'll see how hard that is.