I happen to have this problem, where not all proxies are mirrored correctly. I have a bunch of Aux proxies, only part of which are being mirrored correctly, and with others I have to choose "Mirror Selected" to get it to work.
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Sometimes it doesn't even work with Mirror Selected, but attaching this problematic Aux proxy to a different module helped, then after reattaching it back to the original module kept it working (kept mirroring correctly).