I use both MGIV RC and the Oriental Seagull RCII, and get a good range of contrast with each. My paper was purchased in 2014. I think that the paper referred to in the APUG thread you linked to is also the RCII version.
My safelights are a combination of red and amber LED bulbs bounced off a tan coloured ceiling. I'm using an Ilford Multigrade 400 light source.
My developer is fresh Kodak Polymax T.
I would suspect your developer is the culprit.
Below is a scan of a toned print on 11x14 Oriental Seagull RCII. I used the Ilford paper to print the same negative for my recent card in the current APUG Postcard Exchange.
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