NOTE. The answer below only applies to older builds of WIX and Windows at the time of raising the question. The current version of the WIX service credentials will work if you install your own "NT AUTHORITY" and "NetworkService" (now there is no space).
Original answer:
The correct identifier is NT Authority \ Network Service, not NT Authority \ NetworkService. A space is required, then it works.
It's funny that by default "LocalSystem" works directly with WIX 3.5, but for other well-known accounts you should use the prefix "NT Authority ...", for example, "LocalService" does not work with direct use.
i.e. fix this with:
Account = "NT Service \ Network Service"
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