Same situation for me that Clanger said. I use "Legacy Boot" and still have the Nvidia spiking issue. To be more specific I'm using non-uefi, non-secure boot, and no TPM module installed. As far as I know CSM is the same as Legacy Boot, but I wanted to make note of it in case I was mistaken.
I can't explain why it fixed the problem for you, apart from there's probably more information we need to assess. I've seen just as many anecdotes from people saying that removing their ram and re-seating it fixed the Nvidia spikes, and that doesn't make sense either. So I don't know what's really happening, except that there's probably just bugs in the BIOS and these various tasks act as a workaround to trigger something. I'll give an example of what I mean:
On internet routers and in motherboard bios, after many years of messing with these firmwares, I've settled into a routine where I will first reset a bios to defaults, then update the bios firmware, then reset the bios to defaults again before manually tweaking it. This procedure has fixed countless problems for me over the years, especially in old Linksys routers.
Are you confident that something else didn't fix it, such as disabling virtualition in the bios and/or Windows? When you reset your bios back to defaults is it turning off virtualization, and then are you re-enabling it manually?