Firstly, where is the official bug reporting site? All I can find are dead links from previous discussions.
Anyway, from my MXD I click share as service. This uploads all the layers and their individual symbologies to 'My Content' on my ArcGIS Online account. Integrating this into a Web AppBuilder is relatively simple. The end result is an online map that looks like my original MXD. Great.
However, if I decide to change the order of layers in the MXD's table of contents, then share the map as a service (overwriting the existing one), the legend glitches out on the online map (the Web AppBuilder). Looking down the legend on the online map, the legend is fine up until the first layer where the order has changed.
See below (unaffected layers blacked out. the left shows ArcMap 10.4, and the order I intended for the layers. On the right is the resulting legend in the web map. All jumbled!) nb. this incorrect symbology was not reflected in the map itself, just the legend.