Hmmm... At startup the prechambers, cylinders, cooling jackets are all at ambient temperatures. Only the compressed air and glow plugs are hot. When the glow plugs go off the surrounding metal structures are still cooler. The glow plugs may be masking a small leak, and once the structures get up to temp it masks the issue.
I'm still sticking with Black smoke = unburned/incomplete combustion fuel/oil, white smoke = coolant.

If its running good when its at operating temp then just keep an eye on things, and wait till it gets worst, otherwise do the compression test and report your findings.

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92 300D 2.5L OM602 OBK #59