Which are you experiencing? The video and the description do not match.
In the video, we see 5v is present and the call for 12v is not completed/answered. Could be as simple as forgetting to plug the ribbon to the power button board back in.
In the description, you mention 3 beeps and back off. This is overheating. So the next question would be: How fast is this happening? If it's within the first few seconds of powering on, then the most likely cause would be the APU clamp is either not tight was put back upside down.
If it takes longer than that, immediately following a dis/reassembly, then my guess would be thermal pads were not accounted for. If not, then some of them could've been damaged or misplaced. IMO, the Pros were not equipped with sufficient thermal pads from the factory. Replacing them should be part of any disassembly/cleaning of the system. Especially if it's behaving like it's overheating.
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is the fan and heatsink clean did you put to much/not enough thermal paste? can you open the console and look for damage to anything and post pictures?
Adding images to an existing question
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