The AD5755-1 datasheet (Rev. F) says: "It is not recommended to tie both AD1 and AD0 low when using PEC, see the Packet Error Checking section."
And also (in the PEC section): "It is not recommended to tie both AD1 and AD0 low because a short low on SDIN, that is, a command of 16 zeroes, may possibly lead to a zero-scale update for DAC A."
(1) Zero-scale update(!) refers to the accessible DAC A Data Register and not to the internal DAC A DAC Register, i.e. the analog output?
(2) Can this "errata" only occur with the PEC option being enabled? Even without using the PEC feature there maybe 16 consecutive zeroes on SDIN.