Why does my Netgear Smart Switch randomly untag VLAN on ports?

I have a NETGEAR 48-Port Gigabit PoE+ Smart Managed Pro Switch with 4 SFP Ports (GS752TPv2) that has been sporadically untagging a VLAN on ports. The VLAN in question is an Auto-VoIP VLAN. The workstation VLAN remains tagged on the same ports, and the workstations do receive a DHCP address and full network connectivity via Ethernet passthrough. The firmware of the switch is at the current release version 6.0.0.45.

I first noticed this on a single port a week ago, I re-tagged that VLAN on the port and everything seemed fine until today when a few ports were showing the same issue. I cannot reproduce the problem by rebooting the switch, it seems to just happen over time. Any ideas are appreciated.