Dynamic sampling seems to have been broken since alpha 16. When you zoom into a down sampled image, it will up sample no problem, but when you zoom out again it is expected for dynamic sampling to read the image from memory and resize it / down sample it back down again but that seems to consistently fail despite the logs stating an attempt to resize / down sample.
To reproduce, you need an image that is significantly larger than your monitor and you need to enable dynamic sampling and monitor down sampling in your config.toml.
This might be a really hard one for someone to solve as how I see it so far, the debug logs are lying 😭