When I hit “open in” and then choose “Maximum Available” the resulted image seems to be low resolution, I’m guessing it’s actually exporting the Smart Preview instead. The resolution of the image is 2560x1707 when the full Raw file actually is 5257 x 3505.
Which version of Lightroom uploaded the original? If it was Lightroom Classic or Lightroom CC 2015, then only a smart preview would have been uploaded, so the maximum available in the cloud would be 2560px.
It's been a while since I've uploaded it, I can't remember, but I'm guessing it was CC because it's a Smart Preview. Just last week using the same album, I've been able to export to other iOS apps with the Maximum Available resolution. Today I can't. The funny thing is if I choose "Save to Camera Roll" I get the full resolution of the photo, but when I hit "Open In," I get the Smart Preview resolution. So my current work around now is having to save it to the iOS camera roll then import it into another app, whereas before I could just choose "Open In" for another app and it would open at full resolution. The app I tried this was Affinity Photo.