Why I am asking about AC power:
Your symptoms match what can happen to a desktop Mac when its PRAM Backup battery has become exhausted. [That is not a definitive diagnosis, just a correlation.]
Desktop Macs have a small always-on power supply, that is used to maintain system parameters such as System Time and 'where is the boot drive'. These parameters make startup fast and easy. These parameters are important enough that a PRAM backup battery -- a 3 Volt non-recharging coin cell battery is also available, to make certain the parameters survive across power outages.
Provided you supply power to your Mac at all times, that PRAM backup battery will typically last for as many years as your Mac and be trouble-free.
If you do not supply AC power to your Mac overnight, within a few years that PRAM backup battery can be exhausted, and your Mac may refuse to start because the battery is too low and the parameters have become too corrupted.
--------
A common debugging step when this used to happen in Intel Mac Pro towers was simply to replace the PRAM backup battery. Unfortunately, on the Mac Studio, that is a 25-step tear-down, including removing the power supply and other components, to get to the battery.
For apple-silicon Macs, Apple says a PRAM reset is not needed, and that likely means that PRAM Reset can not be used for additional diagnosis.
-------
if you need to submit your Mac for service, you must sign a paper that says you ALREADY have all the backup copies of your FILES you will ever need. Especially if major components are replaced, your files are NOT guaranteed to be returned to you.
¿What is the date of your most recent backup, and by what method?
