This is bug #8596 [1].

I'm fairly confident I found the cause and solved it with DMD PR #3844 [2] (merged and included in v 2.066-rc2 onwards.)
I did not confirm/show that this was the root of the bug as there was no motivation/incentive for me to do further work after having found what seemed to be the cause.

If this bug is occurring with DMD 2.066-rc2 or later, please re-open ticket #8596.

EMSI had been running into this bug, I don't think it has re-occurred for them since updating to 2.066.

[1] https://issues.dlang.org/show_bug.cgi?id=8596
[2] https://github.com/D-Programming-Language/dmd/pull/3844
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