normal, most leak a little when they're unbolted.

Paul, would simply loosening and removing the bolts allow for a slight decompression of the cases to cause a slight leak? After all you mention that there is no leak when things are snugged up. You appear to have had the engine in and out a few times, which the majority of owners don't. Perhaps the designers expected fresh gaskets and sealant each time the engine comes out as this is mainly for major work. This is a non mechanical viewpoint.

It is a common issue.  When the engine is removed from the frame the engine cases can weep oil from the front bottom, and it stops leaking when the engine is bolted back in the frame and the lower engine mount bolts are tightened.  Nobody designed it that way.....but that is what often happens when you take the lower engine (peg mount) bolts out....even if the engine is still in the frame.

You might want to drain the oil while the engine is out of the frame, or put a bolt through the case and tighten it up until you put the engine back in the frame.