Is it the EFIS control panel that determines which side to take primary data from or the selection of the FD switch; the 1st one selected being MA? Usually, in pre-flight the PF selects their FD first to make their side the master. Would this not be the factor that determines which side all the parameters are taken from? Thus PF, with FD = MA, and with a failed EFIS panel will also lose more than PNF. However, why just the ALT? If the AFDS or FCC decides there is inaccurate data then why not remove all parameters that can feed into the AFDS system e.g. IAS, HDG, VSI?
It's a question not an answer.