I wonder if anyone here might have an insight into why this occurs:

I am laying up type in the cyrillic alphabet and I am getting different characters depending on the font I use. As you can see in the attached screen shot, Helvetica Neue (lower) displays the й properly, but Ropa Soft (upper) shows it as a 'u' with lower half circle on top. 

My initial thought was that it had something to do with the case, which does correct the characters. But then I am left with the whole phrase in upper case, as opposed to the 'semi upper and lower case' as helvetica shows (in latin terms). 

Any ideas are much appreciated! Thanks :)

It’s simply a stylistic variant of the lowercase |и|, modelled after the italic form, much like our single-storey |a|.

I’m not a reader of any language written in Cyrillic, so I can’t say how common or unusual it is.

For what I can recall, Bulgarian Cyrillic tends to use a number of letterforms modelled after italics. And Ropa’s designer was indeed born in Bulgaria.

I remember someone complaining that these variants aren’t really historical, but rather the fancy of some foundry, which gained political traction.

Thanks for the info Riccardo, very insightful and interesting to now go hunting for more! Cheers 

In handwriting this character is more like a Latin "u". This particular font uses the handwriting convention rather than the typographic one, but it is the same character as the one that looks like a backward "N". You find it more often in italic fonts, but it is perfectly ok, not a coding error.

More on Bulgarian Cyrillic​.