The Cyrix is a bi-sensing device so will sense voltage on either bank.
My guess is this is possibly another situation of relay cycling due to Victron giving instructions for installation
which suggests connecting all charge sources to the start battery... I fix these situations with a fair bit of regularity..
Connecting to start first is bass ackwards on vessels with large house banks that routinely get deeply cycled. This can cause the relays to cut in and out, in sand out until house bank voltage is finally high enough to remain latched.
This whole fear mongering campaign started with Charles Stirling scaring everyone into believing you "must" charge the start battery first... He did this because he had a product to market... Course this is an entire topic unto its own..
The only other issue could be in the neg for the relay or a fuse in one of the pos leads.. Be sure the neg is connected to the 86 terminal and has continuity to battery negs...
Yandina and Blue Sea ACR's are perhaps the most reliable pieces of electrical gear
I install. The Cyrix never wowed me much but they are simple devices with most issues related to installation