Screenshot: https://i.imgur.com/5WdSbJD.png

Running Paper-225 for MC 1.13.1. Using version of WorldBorder compiled from Github source on 8/31/18.

Previously reported and closed by the reporter; seems like it's a 1.13 bug not specific to WorldBorder's Fill.
#98

I can't seem to replicate it in a single-player vanilla world. I also can't find any broken trees in a world that I filled using regular Spigot. Might be a Paper-Spigot issue?

Edit: So far, no issues with fill on regular Spigot. Paper-devs say it does happen on regular Spigot too. Seems to be very inconsistent.

Edit 2: On the server that had the issue, I just flew outside of the area that was generated using /fill and I'm not seeing the same problem. Either a recent Paper-Spigot fixed it, or WorldBorder makes the bug much more likely to occur?

@eightbitx
https://bugs.mojang.com/browse/MC-125007 This is the bug.

@TheCompieter Thanks, I saw it. During my testing I was unable to find any broken trees in vanilla nor in a world that I /fill(ed) with regular Spigot. I'm guessing this is a vanilla bug, but whatever code PaperSpigot changed made it much worse. Interestingly, even with Paper the issue seemed to only occur during /fill - but, since it's not a WorldBorder bug I'll close this.