My situation was a bit different than yours in that Iberia would not let me select a seat for free, which I should be able to do due to my Oneworld status (Sapphire).
On the JAL site I could delete my Frequent Flyer number and then re-enter it.
Then I could select a seat for free using the RJ site.
And that was reflected on the IB site and so far, it has held.
It's all too weird!