That's odd. It's as if they have forgotten to put their chlorine interference remover into this batch of R-0004.
I usually test with a meter, but I just took a sample of my pool water that sits currently at FC 11ppm. My pH is around 7.8 right now, so I first added some acid demand drops to take pH down to about 7.4 to make a drift more visible and then added R-0004. Even though my FC is slightly above 10ppm, the test worked fine, no drift noticeable.
At FC 5ppm, your R-0004 should work absolutely fine, without any drift. I'd say there's something wrong with it.
As a work around, so you can close your pool with peace of mind, there is a trick to reduce the FC of a sample without changing the pH: Make a 1:1 dilution with distilled (or deionized) water - this is important, tap water doesn't work. Because distilled water has no Alkalinity (that's reason 1 why tap water doesn't work, tap water has Alkalinity), but your pool water has (which means that it is buffered), the dilution will not significantly change the pH of the sample, but it will half the chlorine (that's reason 2 why tap water doesn't work, tap water is usually chlorinated). Let's hope that your R-0004 can cope with FC 2.5ppm.
And then I'd order another bottle of R-0004.