Omega Speedmaster x-33 Gen 1 glitch?

Had my X-33 serviced by Omega in 2016.

Battery ran out recently so I did a battery change, no problems with that.
Put it all back together and set the time and date.

No problems.

Tested the backlight and it immediately went into 5 second EOL jumps 😡.

Removed the battery again, let the smoke disperse and refitted the battery etc and set time and date, no problems.

Watch is working fine and I haven't tested the backlight again.

Has anyone had this issue or know of a likely cause?

Don’t let the smoke out!!! 😀

Did you test the battery before you installed it? I always do, because depending on where you get your stock from, the battery may not be terribly fresh to start with. When you use the backlight it puts an additional drain on the battery. so if it drops below the voltage that triggers the EOL indicator, then that's why it went into that mode. EOL is tripped at voltage below 2.6V on this one.

Ideally, you should. But even using a simple meter will give you some information...