You don't actually need to render.
And once a clip has been rendered (green bar above), it should not need rerendering if you trim the clip shorter. If you trim it longer, only the new parts should have the red bar.
Yes, that's exactly the problem I'm having here. With all my HD projects it works that way, but not this one with PAL footage. It used to happen in the past too sometimes when I worked with PAL more often, and there was some kind of trick to avoid it, but I've forgotten. It's really frustrating, you don't just have to render if you trim it longer, also if you trim it shorter, and the whole clip needs to be re-rendered, not just the added frames.
That's very strange behavior. I've not seen that myself.