In TwinCAT I have had the same problem, but with two global variable lists.

The issue is the order of the initialization of function blocks or global variable lists. Your variable Z_A1_X2_Ethernet is most likely initialized after your fb: Fb_ModbusTcpSlaveDiagnostics.

TwinCAT gives the following instructions on how to explicitly set the order: https://infosys.beckhoff.com/english.php?content=../content/1033/tc3_plc_intro/2529649291.html&id=

In my problem, I used a variable from one global variable list to initialize another variable in a different global variable list.

My solution was to use {attribute 'global_init_slot' := '49992'} at the top of the global variable list that needed to be initialized first and {attribute 'global_init_slot' := '49993'} in the second global variable list.