It just happened again. It was so bad I couldn't even attach to the container it was running in. But I noticed the container was using all of its allocated memory (8GB). I increased it to 12GB and the instant I did, attaching to the container worked, but it immediately used those new 4GB as well. But a few seconds later, both memory and CPU usage dropped to almost 0 again. So it seems there is a bug in photoprism that triggers endless 100% CPU usage once memory runs out. Not great :D