Ugh. Mea culpa.
I am now not sure that the forward declaration issue was causing the SpawnActor failed message.
I was seeing a crash due to the forward declaration for a UPROPERTY issue. And just above the crash I had seen the message, and (most likely) leapt to the wrong conclusion.
So after a time I noticed that I was still getting the message about SpawnActor failed. Just because there was no crash to pause the log, I had not seen it.
However I noticed that it was there while debugging another issue, went hunting and tried the fix mentioned in this thread: and it fixed the problem.
Sigh.