Each flipper should have its own EOS switch.
Four flippers, four EOS switches!

On classic Stern games (if you have an upper right or left flipper) there will be an extra set of EOS contacts that are normally open (N.O.) on the lower flipper.

Let's just take the right side, as the left is the same. The lower right flipper will have 2 sets of EOS switches, one N.C. and one N.O. The upper right flipper will just have one EOS switch that is N.C.

The N.C. switch on each flipper is a standard EOS that controls the power/hold coil for that particular flipper. The extra N.O. switch on the lower flipper controls power to the upper flipper on that side of the game. In other words, the upper right flipper will not receive power until the lower right flipper has reached full stroke causing the N.O. switch on the lower right flipper to close.

Not sure why Stern did this, but my guess is to limit the current draw from two sets of flippers energizing simultaneously, blowing fuses, etc. In this scenario, the upper flipper engages after the lower has reached end of stroke (or at least until the N.O. set of contacts close). It's not noticeable at all fortunately.

Stern Lightning has 4 flippers; so 6 EOS switches.

As far as your particular problem, follow all the connectors that are shown for the left flipper, check the left cabinet switch, check the voltages, etc. There is a flipper enable relay on the solenoid driver board (SDB) that could have a bad solder joint or one set of contacts, for the left flipper, might be bad. The connector on the upper left of the SDB controls the power to the cabinet switch and flipper coils, check that as well as pin continuity. Also run continuity checks for all the wiring. I've run across broken wires before inside insulation. Lastly, check that all the wiring is solidly soldered and wired correctly.

Yep the board in the upper right. That provides the HV for the displays and also feeds the solenoids through all the transistors along the bottom edge. It also has the flipper enable relay along the left side.

Bender rules!!! "You can kiss my shiny metal ass!"

Looks like a trace burned open. I've seen this happen if a coil gets locked on and doesn't blow a fuse but takes out the trace instead.

You can jumper a broken trace but I'd double check everything in that circuit.
There's probably a reason why that trace got stressed.
Could have been a past issue that someone already addressed (but failed to notice the stressed-out trace)
Refer to your wiring diagram!

Glad you got it working!

Could you pot a pic of your lower right flipper coil wiring? I need to reconnect a new coil, but there's like 5-6 wires that were cutvoff the old one when it got removed, and none are labeled as to where they came from.
Thank you
Jeff