Kai Hao’s suggestion of adding is-style-wide
class name to the separator worked, and did restore the lines to their previous width.
However, this is only a patch and not a true fix. I say this for 2 reasons.
(1) The visual editor presents the option of choosing “wide-width” or “full-width” for the separator line. Therefore, the underlying code needs to follow through and display the separator line as selected (wide-width or full-width).
(2) Put simply, it worked perfectly before the upgrade, so *something* changed to make it not work. Something small was missed in the 5.8 update.
This can (and should) be fixed; adding code in the code editor every time, as per Kai Hao’s remedy, is not something I’m interested in doing, especially considering reason #1 above.