To reproduce:
Use the style for the Journal of Peace Research
https://github.com/citation-style-language/styles/blob/master/journal-of-peace-research.csl
which has collapse="year-suffix" cite-group-delimiter=", "
and <layout prefix="(" suffix=")" delimiter="; ">

Test with any two items from the same author and year (e.g. a simple duplicate).
Expected: (Smith 1776a, b)
Actual: (Smith 1776a; b)

Oddly, this does seem to work correctly with collapse="year"