I've just upgraded from 5.x to 20.1 (whew!) and found one application that is displaying changed behavior, and I'm pretty sure it's a bug.
I have a select list that allows multi-selection, with about 5 values and 3 selected. When it's displayed in read-only mode, there are three items displayed, but the display value for the first item is repeated. When it's non-read-only mode, the items display as expected.
Demo here: Select List, Read-Only
Can anyone suggest a workaround for this? If this is a bug, and is fixed in Apex 21.1 or whatever, I'm not able to upgrade past this version because of the removal of IE 11 support.