FD engagement
What you read is true and it exists....you will find it in the FCOM 1 (also known nowadays as DSC, it is under AUTOFLIGHT / FLIGHT GUIDANCE / FLIGHT DIRECTOR.....its a note in the flight director automatic disengagement and reads as follows:
If AP/FDs are OFF, and FD2 then FD1 are engaged within 180 ms (one computation cycle), a flip flop of master FMGC may occur.
As a result, no vertical mode engages, the FMA 1st column, 1st line (A/THR mode) is blank or displays dashes.
Engaging V/S mode manually re-selects the correct FMGC and restores the display.
now for the explanation....flip flop means a reversal in the function of the FGMC concerning the MASTER-SLAVE logic between FMGC1 and FMGC2....meaning that that FMGC1 goes from master to slave and back, and the opposite occures with FMGC2 (slave to master and back).and as a concequence, the Aircraft system gets confused and doesn't know which FMGC is MASTER and should control the A/THR which merely inhibits the engagement of normal vertical modes and auto-thrust, that's of course confusing and inconvenient for flight crews, and even sometimes hazardous depending on the flight phase this flip-flop occurs.
Its worth noting that flip-flops do not cause loss of data from FMGC.
hope this was of help.
cheers