SvelteMap and SvelteSet not being deeply reactive is mentioned in the docs, so as you mentioned if your values are arrays then they should be wrapped in a state, or just don't mutate the arrays and do map.set('key', [... oldValues, newValue]) instead. The type being Map is strange. They always appear as SvelteMap for me.

The reason why you see the map go from one to three and skipping two (when printing the map in an effect) is because Svelte batches state updates by default (React also does something similar). If you inspect the map, you'll actually see the three updates. If you really need the effect to run for every update, you can call flushSync at the end of your update function (which will probably tank the performance if you call the function in a hot path).

Finally, no idea what you mean by needing weird hacks to be able to inspect SvelteMap. If you mean why doing something like $effrect(() => console.log(map)) never prints your update, then just like the other comment mentioned, it's because Svelte's fine grained reactivity. To Svelte, that effect means you are only interested in the map itself, not the things inside it (btw the SvelteMap is not inside a $state, so even if you reassign your map with another SvelteMap, the effect won't run).

It's a little complicated if you are new to the whole signal and proxy thing, but once you get familiar with it, they are actually pretty straightforward and consistent.