Bingo! You were on the right track with the lamp signals to the LDB being the issue and glad you found it.
I'm going to get technical here and hope it makes some sense.
With a bad connection on the lamp AD1 signal, the following lamps below will not work:
"uu" indicates the decoder chip number
"bbbb" indicates the binary output to select on the decoder
"dd" is a decimal representation of the binary output selected
A binary logic '0' is essentially 0 volts and indicated as Low on your logic probe.
A binary logic '1' is essentially 5 volts and indicated as High on your logic probe.
The Lamp Address signals AD3, AD2, AD1, AD0 are respectively represented in the binary output number. The second binary digit from the right is lamp address AD1. Anywhere where AD1 cannot change to a logic level of 0 (low) is where that lamp will not be able to be selected. i.e. the second column from the right in the binary numbers listed are where that signal needs to go low (logic 0) in conjunction with other lamp address signals in order to activate those lamps. And your AD1 is stuck high (logic 1) so wrong or no lamps will be selected.
uu:bbbb dd
------------------------------
U1:0000 00: Collect Bonus 1000
U2:0000 00: Collect Bonus 2000
U3:0000 00: Collect Bonus 3000
U4:0000 00: Collect Bonus 4000
U1:0001 01: Collect Bonus 5000
U2:0001 01: Collect Bonus 6000
U3:0001 01: Collect Bonus 7000
U4:0001 01: Collect Bonus 8000
U1:0100 04: Right Spinner center 400 Lamp
U2:0100 04: Right Spinner lower 400 Lamp
U3:0100 04: "M" upper rollover lane and "M" right side standup target
U4:0100 04: "G" upper rollover lane and "G" left side standup target
U1:0101 05: --
U2:0101 05: --
U3:0101 05: --
U4:0101 05: --
U1:1000 08: 5X Bonus Multiplier
U2:1000 08: 4X Bonus Multiplier
U3:1000 08: 3X Bonus Multiplier
U4:1000 08: 2X Bonus Multiplier
U1:1001 09: Left and Right flipper return lane 8000
U2:1001 09: Left and Right flipper return lane 6000
U3:1001 09: Left and Right flipper return lane 4000
U4:1001 09: --
Backbox:
U1:1100 12: Ball In Play
U2:1100 12: Highest Score To Date
U3:1100 12: Game Over
U4:1100 12: Tilt
U1:1101 13: Number of Players 1
U2:1101 13: Number of Players 2
U3:1101 13: Number of Players 3
U4:1101 13: Number of Players 4