Not mentioned yet, and I don't want to muddy the waters, but the inlet manifolds (more specifically the gaskets) can leak, they have coolant flow.
Also not wanting to spread gloom, but when I had a similar issue if turned out to be coolant getting sucked into the centre front cylinder via the head gasket.
Unfortunately a number of possible causes - the bleeding is a good place to start I reckon. I think at some point some folk were using a pela vac kit to do it? (or have I made that up?)
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