My suggestion would be to check the send/receive midi channels of both units again.
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The A4 has a so called “auto channel” in the <Global Menu/Midi Config/Midi Channels>. If the OT sends to the same channel, which on the AR happens to be the “auto channel”, you could have an unwanted “talk-line” between both.
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In the menu <Midi/Control> of the OT is a feature to activate/deactivate CC and note in/out and to limit this to int/ext/int+ext. Maybe that the fader is using this way, but I didn’t check with the midi implementation mapping.
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The Elektron units send/receive NRPN too - not only CC. Did you check this?
Hope this provides a kind of starting point… 