By clicking on "x in y out"-button in Reaper inserts dialog you can follow how things are routed.
It could be you have a mono->stereo component in the chain.
At least the final one could be this way - but if mixed may cause unexpected results.
If eq is mon-stereo then the rest are to be stereo components. At least I think it would be beneficial.
Waves separate this with separate components very much, mono-mono, stereo-stereo, mono-stereo.
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-- Windows 11 Pro, i7-12700F 2.1GHz 32G, RME Digiface USB Audient ASP800 Lexicon MX200, Reaper 4.78 --
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