The problem here is that according to Amex MR, e.g. in the US, this is actually permitted, but it goes against KF rules.

Add that certain bloggers even state explicitly that it works (like Sebastian on Loyaltylobby just the other day) and is fine / allowed which is not true any longer as OPs and several other recent experiences show.

Now will everyone get caught? Probably not, but risky and surely a headache as KF has susoended such rewards tickets before and at times been pretty hard nosed about it.

Best action seems to be to ASAP transfer the needed miles from one of your accounts and plead your case telling truthfully that you were not aware. YMMV