Why am I getting unusually high CHT on ground operations on one engine in a Seneca II?
If the gauges are good (swap them to see if the problem moves to the other engine), the baffling is good (baffles must be tight to the fins to force air between them with no leaks around the plenum), the cowl flap works normally (not being unknowingly blown partly closed by loose linkage), the most likely cause is an intake leak that's causing the cyl with the probe to run lean.
Either the rubber intake hose is bad, the gasket at the cylinder is leaking, or the sheet metal intake tube to which the hose connects, that is swaged into the intake spider tubes at the oil sump, is leaking.
This is an old problem on Lycomings. The little sheet metal pipe stub  is installed with a rolled bead that engages with a female groove in the hole in the sump, and depends on interference for a seal.  They can lose the interference and start to leak (you will be able to move it if you loosen the rubber hose).  There is a (very expensive) swaging tool (I believe you can rent from Lycoming) that re-rolls the bead from the inside.  Some people prefer to just remove the sump and send it somewhere to be done.
Although the redline is there, you really want to keep CHTs below 400F if at all possible.