In «فِرْقٍ», a still ra follows kasrah and is immediately followed by qaf, an emphatic letter. The general rule points toward heavy ra. However, because the following qaf itself has kasrah, the transmitted discussion also recognizes a light rendering.
Thus both tafkhim and tarqiq are reported for this particular instance. It would be inaccurate to present the ordinary emphatic-letter rule as if it admitted no such detailed qualification.
These fine differences should be practiced consistently with an authorized reading and a qualified teacher. A special exception does not abolish the broader rule.