I haven't seen the bird so it is hard to tell, and field experience is much better than opinion on picture. So if the bird looks so rufous in the field, then I agree that it is very odd for Swinhoe's Plover.
But to me the eyering contrast to much with the head part, where it seems to show very little contrast on Japanese.
There is a blurry paler area just above the bill continuing into the bill (thin pale stripe) like Swinhoe's and I can not find any pic of Japanese suggesting that.
The lore seems to very slighty contrast, and are delimited by this pale stripe above and by the throat below, this area seems to be less or more dark from many pics of Swinhoe's can be found in the net.
Ear coverts show a few paler feathers as many birds shown on OBI.
Japanese Robin, in both sex show a plain head so why first winter (I'm not saying it is a first winter) would show such an head pattern, that fits in many aspect Swinhoe's Robin. They are just very poorly marked.
The apparence of the throat just depends on the position of the feather as shown in pics 1 and 2(looks more white on pic2).
Japanese robin show also some contrast between rectrices and mantle... and this contrast seems to be variable in Swinhoe's.
The scaly underparts with grey washed on the flank is typical of Swinhoe's.
So the only odd thing with this bird are the very rich upperparts to me...
But I agree that it seems that there is very few picture of the 1st W Japanese Robin, but I don't know why, I'm very little confident that it is one but very happy to be corrected.
All the best,
Jonathan
