What you're doing should work. I'm currently working on a new plugin (in Elgg 1.8) and everything works fine.
When developing a plugin is't always a good idea to disable caching, as you don't have to worry about issues.
Please keep in mind that Elgg 1.9 is still in heavy development and some things could not work. Maybe you could report this issue in github (https://github.com/Elgg/Elgg/issues) so the developers can tell you what's going on.
Thanks Jerome, I'll try to see if I can replicate this issue in a test plugin. If I encounter the same behaviour, I'll file a report.
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