Just tested your case with version 1.20 and everything is working fine:
Node-RED for ctrlX 1.20 - Subscription to analog ctrlX IO output
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For version 2.4 it could be related to a known bug. Please enter the default values (instead of leaving it empty) in the subscription configuration:
Node-RED for ctrlX 2.4 - Subscription to ctrlX IO analog output
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