I wanted to jump in and share my experience.
I installed my FW 12gen board in the case, with an antenna and a Wi-Fi card. Plugging in the power would get the system to post. Pressing the power button would cause the system to lose power immediately. The LED status lights would immediately turn off. Replugging in the power would cause it to POST, but hitting the power button would never start a boot cycle. I tried plugging in the power on each of the ports as a test. All reacted the same. I took the MB out of the case and everything works. I verified I was in standalone mode, and nothing looked strange to me. I plugged a laptop screen and a battery into the MB to test the system, and it worked without issue. I toggled standalone mode off, then back on to try something. I removed the screen and battery, placed the MB back in the case, but didn’t put the top on. It worked just fine, but the case intrusion lights blinked. I put the top back on, and the power-off issue came back. After a little on/off testing, I realized the case was screwing down the power button a little too tightly. I loosened the screw around the power button, and everything works fine.
Just wanted to share in case someone else goes down the same troubleshooting route, thinking they have hardware issues.