GUACAMOLE-176 added support for layer opacity to the guac_common_surface object. Although layers always default to opaque, the opacity property of guac_common_surface is not properly initialized to fully-opaque (0xFF). Layer opacity is not sent to the original user of a connection unless it changes, so the original user is unaffected, but any users which join a shared connection will be synced with display state, including the incorrect opacity. For VNC and RDP, only the default layer is used and this bug has no effect, but SSH (and telnet) use multiple layers.
Bad commit identified by git bisect: 6bd19b6