multiple shortcode tags only every other parsed

I’m writing a plugin that will use shortcodes and have the following tag structure:

[module][item]1[/item][item]2[/item][item]3[/item][item]4[/item]
[/module]

the problem is that only items 1 and 3 are parsed (sent to the handler function) but items 2 and 4 are ignored. However, if I put spaces or line break between the items, e.g

[module]
[item]1[/item]
[item]2[/item]
[item]3[/item]
[item]4[/item]
[/module]

everything works are expected (apart from the annoying br or p tags but that’s a different story).

Is this by design or a parsing bug?

thanks