With external plugins DND doesn't fully work. The possible targets are only next to internal plugins, so you cannot e.g. drop an external plugin between two other external plugins. Note that this is not a regression but was already the case with the 4.12 panel and gtk3 plugins.
Peter de Ridder referenced this bugreport in commit b3ccde3fc444a4f590574e24c0f7fdce8e6aaf17 Fix DND between external plugins (Bug #13687) https://git.xfce.org/xfce/xfce4-panel/commit?id=b3ccde3fc444a4f590574e24c0f7fdce8e6aaf17
This works nicely now, we should probably consider backporting to 4.12.