The reason for this is that the suffix isn’t part of the citation-label. You can notice that the <citation> for this style has two important components (besides the optional supplement / locator): the citation-label and year-suffix.
If your bibliography style does not add year-suffix explicitly, it does not get rendered. It seems that your style chose to add it at the end of the entry in this case, which is why the letter appears there.
There is more info, including a workaround, in this post: Why does Hayagriva with this .csl file not produce year disambiguation?
(I think we should eventually merge those threads to keep it clean…)
I’m not sure if we can consider this a deficiency in the CSL spec or a hayagriva bug, but it is worth an investigation. Unfortunately, I haven’t found much info on how other processors handle this yet, but I’d appreciate further contributions here.