Posts from sdf not making it to lemmy.world

Hi,

Several posts I've made to /c/fountainpens have not made it to lemmy.world from the lemmy.sdf.org instance.

If you compare these two urls, you can see that the top post (and several others) are only visible when viewed from sdf:

https://lemmy.world/c/fountainpens https://lemmy.sdf.org/c/fountainpens@lemmy.world

I've looked at the debugging tool and the sdf instance thinks that outbound lemmy.world is up to date, so has lemmy.world rejected the posts somehow?

It seems to happen non-deterministically. Some posts have made it through. This has been ongoing for at least a couple of weeks now.

Any ideas?

Edit: this post did federate... could it be to do with posts with images or something?