It's the RAM. It needs to "trained" which takes some time but for for some reason these boards seem to randomly forget their training, requiring it to happen again.

I've never had memory training be forgotten with my AM4 nor LPDDR5-based laptops and NUCs. Is this a new thing with AM5 or something? Or just a certain brand of BIOSes?

It's a common issue on consumer boards with DDR5 and more than two DIMMs installed.

Doesn’t affect soldered memory or lower speed memory (like DDR4). Many memory controllers fail to achieve good speeds and timings at all on 4 DDR5 DIMMs, and fall back to running DDR5 at 3600MHz instead.

Ok, so user selects too-high speed, controller tries for ages and fails, but doesn't save since it's overridden by user in BIOS?

I distinctly recall thinking my LPDDR5 NUCs were broken since they seemingly didn't boot the first time, until I recalled the training stuff. Took up to 15 minute on one of them. But neither has had any issues since, hence my question.

Wonder if DDR5 ECC ram has the same problem? I'm meaning the real ECC stuff, not the "on chip only ECC" that all DDR5 has.

The controllers which support ECC are usually a lot better and able to handle more channels. They also typically require active cooling.

huh, its been a decade since i built a PC, whats changed?

DDR5 is much, much more fickle than DDR4 and earlier standards. I think it's primarily due to pushing clock speeds (6000 MT/s would be insanely fast for DDR4, but kinda slow for DDR5).

Memory training has always been a thing: during boot, your PC runs tests to work out what slight changes between signals and stuff it needs to adapt to the specific requirements of your particular hardware. With DDR4 and earlier, that was really fast because the timings were so relatively loose. With DDR5, it can be really slow because the timings are so tight.

That's my best understanding of it at least.

It's an AMD thing

My guess is bigger numbers, higher voltages, tighter timings.