My LVAR assignments worked 99% of the time and by chance I received an error that looks like a timing issue:
FSUIPC7.ini
[LvarOffsets.Fenix]
1=L:S_FCU_EFIS1_FD=UB0xA000
2=L:S_FCU_EFIS2_FD=UB0xA001
3=L:S_FC_THR_INST_DISCONNECT1=UB0xA002
4=L:S_FC_CAPT_TILLER_PEDAL_DISCONNECT=UB0xA003
[Keys.Fenix]
8=N123,8,x0100A001,x01,x0100A001,x00 -{F12: Press=offset byte set, offset A001, Release=offset byte set, offset A001 }-
11=N122,8,x0100A000,x01,x0100A000,x00 -{F11: Press=offset byte set, offset A000, Release=offset byte set, offset A000 }-
12=121,8,L1:R,0 -{F10: Press=Lua rmp_iface }-
[Buttons.Fenix]
2=PA,0,Cx0100A002,x01 -{offset byte set, offset A002}-
3=UA,0,Cx0100A002,x00 -{offset byte set, offset A002}-
4=PB,0,Cx0100A003,x01 -{offset byte set, offset A003}-
5=UB,0,Cx0100A003,x00 -{offset byte set, offset A003}-
Log File:
52125 -------------------- Starting everything now ----------------------
172125 G:\MSFS\Community\fnx-aircraft-320\SimObjects\Airplanes\fnx320\aircraft.CFG
172141 Sim stopped: average frame rate for last 127 secs = 33.0 fps
172141 Max AI traffic was 0 aircraft
172141 -------------------------------------------------------------------
172172 Starting WAPI...
172172 [INFO]: **** Starting FSUIPC7 WASM Interface (WAPI) version 0.5.10 (WASM version 0.5.9)
172172 [INFO]: Connected to MSFS
173516 Maximum number of custom events available is 1024 (defined by ini parameter MaxNumberOfCustomEvents)
173531 Aircraft="FenixA320_DLHDAIZI"
173563 [INFO]: SimConnect_Close done
177734 C:\Users\holger\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalState\MISSIONS\Custom\CustomFlight\CustomFlight.FLT
183203 Starting WAPI...
183203 [INFO]: **** Starting FSUIPC7 WASM Interface (WAPI) version 0.5.10 (WASM version 0.5.9)
183203 [INFO]: Connected to MSFS
187391 #### ERROR: No SimConnect data seen for more than set stall time - will attempt to reconnect...
187391 **** SimConnect Data Stalled! Re-connecting now ... ****
187641 SimConnect_Open succeeded
187641 Running in "KittyHawk", Version: 11.0.282174.999 (SimConnect: 11.0.62651.3)
187641 MSFS version = 11.0.282174.999
187641 Initialising SimConnect data requests now
187641 Maximum number of custom events available is 1024 (defined by ini parameter MaxNumberOfCustomEvents)
187656 User Aircraft ID 1 supplied, now being used
187656 SimObjects\Airplanes\fnx320\aircraft.CFG
187672 User Aircraft ID 1 supplied, now being used
188375 System time = 29/06/2022 18:58:52, Simulator time = 18:58:42 (16:58Z)
188609 **** Lvar 'S_FCU_EFIS1_FD' not found: cannot add to offset 0xA000
188609 **** Lvar 'S_FCU_EFIS2_FD' not found: cannot add to offset 0xA001
188609 **** Lvar 'S_FC_THR_INST_DISCONNECT1' not found: cannot add to offset 0xA002
188609 **** Lvar 'S_FC_CAPT_TILLER_PEDAL_DISCONNECT' not found: cannot add to offset 0xA003
188625 Lvars/Hvars received - checking aircraft autos....
2184703 Sim stopped: average frame rate for last 1997 secs = 32.9 fps
From the time stamps it looks like LVARs were received a few milliseconds after the INI file was parsed.