This has been my experience with cedar as well. It soaks up ground moisture like a sponge and pretty soon, the fasteners aren't really that tight anymore. The other issue I experience is the various joints and crevices in the chairs, were pollen and leaves and whatever, accumulate in there and hold moisture. I guess you could mitigate this by keeping the furniture on something like concrete or pavers but for those of us who have our A-chairs on substates like decomposed granite or just the bare ground, or under trees that shed a lot, it's been an issue for me.
I know this is not probably where the OP wanted to go but if I wanted a set of A-chairs that were going to last, it wouldn't be wood. There were some recycled plastic ones at the local garden store that came in all sorts of colors. They were about $300 each but honestly, they will outlast any wood chair and you can just hose them off. For, that would be worth it.
Erik