Replaced logic board but now Mac won’t run from battery

I have just replaced the logic board on my MBP after the previous one failed. I had it diagnosed at an authorise apple repair centre who confirmed this was the issue. Rather than pay the high price for them to fix I have opted to do it myself using a second hand logic board.

I have installed the board using the iFixIt instructions and all seemed fine, the machine powers up and runs correctly. The only issue I encountered at first was that I had to format the ssd and use recovery mode to reinstall MacOS. However, after running it for a while I unplugged from the AC adapter and the Mac immediately lost power and shutdown. The battery itself has charged whilst plugged in and shows as 100% charged.

I have tried all the options such as resetting NVRAM, SMC etc, I also opened it back up to check all cables connected properly. I have booted into the Apple Hardware Test and it reports no issues. Battery health shows as normal, cycle count is 134.

The other odd behaviour I see is that it will boot up immediately when plugged into ac power, rather than waiting for me to hit the power button. I have read a lot of posts about similar issues to this which usually recommend resetting NVRAM / SMC but none of these help. I’m pretty sure the machine didn’t behave like this before.

So, I’m trying to check / rule out any other issues before going back to the seller I bought the logic board from. I am wondering if this is likely an issue with the replacement logic board?

Thanks!