It became obvious after enough fratricide shootings of pilots firing at flashes of red. There are examples of Japanese front aircraft with the red outline though.
Here is just a few.
I'm speaking more specifically about 5th AF birds. Sorry I was not clear on that. The Aleutian 38s may have had red around the roundels. Having spend a lot of time on the 54th FS, I don't recall coming across a photo of one. I never trust profiles, and I love doing them for fun

Was there a photo of a 39th 38 with red around the star and bar? I dug through the 39th FS history and didn't find any. I'm not stating there is no chance that one existed, but it was not the norm in the 5th AF
In general, in the photos that are clear in the histories of the 5th AF 38 groups it shows just the white bar, and later the blue outlined star and bar after the short lived red outlined version was dropped. I suppose new production aircraft made when the red outlined star and bar was used may have arrived in theater with the red outlines. But the practice with the 5th was to eliminate them for the obvious reasons.