Zyxel XGS4600 SFP+ modules randomly stop transmitting after firmware 4.50; DDM shows TX power -inf and 0 mA bias

On Zyxel XGS4600 switches, 10G and 1G SFP+ transceivers (including an FS.com DWDM-SFP10G-80) intermittently stopped transmitting; only removing and reinserting the module restored the link. Transceiver statistics at that moment showed TX power -inf dBm, voltage at the 3.00 V low alarm and TX bias 0.00 mA. Port disable/enable had no effect. The problem started after upgrading to 4.50(ABBI.1); downgrading to 4.40(ABBI.2) stopped the drops but corrupted the transceiver information display. Stacked 32F units, especially member 3, were affected most.

• Hardware: Zyxel XGS4600-32F, Zyxel XGS4600
• Transceivers: FS.com DWDM-SFP10G-80, 10G SFP+, 1G SFP
• Form factor: SFP, SFP+

Symptoms / messages:

TX Power(dbm) -- neg inf
TX Bias(mA) 0.00 low alarm
link lost until module reseat