my answer is not ZF specific.
the threaded bore in the aluminum casing will expand when hot and thus "loosing" its grip on the plug. this would be more so if the plug is steel as it does not expand as much.
the reverse should be heeded when reinstalling.
would you mind educating me on this plug. is it a straight thread with a sealing face? if so, is it gasket or oring
the shop manual illustrations look like they may be tappered threads
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