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So I finally decided to take a close look at why my spoiler cracked.
I noticed that the spoiler in down position sits flush to the decklid on the LH side but sits significantly lower on the RH side. Since the crack is only on the RH side, this seems to indicate that the spoiler is sitting low enough that it is bottoming out on the decklid. This could cause long term stress while in the down position. Inserting a feeler into the unseen portion under the spoiler confirmed that I had clearance on the LH side but none on the RH side.
I then noticed that the spoiler mounts had shims underneath them and the RH side had one extra shim. By removing one shim it raised the spoiler up and away from the decklid. So now the spoiler sits more evenly on the decklid and it has clearance underneath it. Not sure if this was the root cause, but if your spoiler has not cracked yet, it might be worth counting shims and checking for clearance between the spoiler and the decklid.
Here are a few pictures I took of the spoiler and shims.
Tom LH side. Body, decklid and spoiler fairly flush to each other. RH side. Spoiler sitting too low. Spoiler shims. Yellow level is pointing to location of shims. Removing shims raises the spoiler.