I am seeing the same thing in my adsense for two of my ad links. I believe, this optimization suggestion comes only for adlinks and yes, I think it's kind of a bug in adsense. Adsense reporting has always been confusing around adlinks.
Probable Reason (my guess)
It's probably because of the way adlinks work. The adlink requires two clicks - one click for ads to be shown and then actual click on any of the ads. Their optimization suggestion engine is probably counting viewability as
Viewable % = (Ads viewed by user for 1 sec) * 100 / (No. of times adsense code is loaded)
For adlinks, adsense code is loaded for all impressions but the ads will be viewed only after 1st click, which will be true only for very small percentage of total impressions (1-2%). So, viewability % for adlinks will be very low in comparison to normal display ads and hence the warning/optimization suggestion.