the simplemde change would be:

diff -r 2f53d41ae71f 
share/roundup/templates/jinja2/html/issue.item.html
--- a/share/roundup/templates/jinja2/html/issue.item.html	Tue Jul 
28 21:41:59 2020 -0400
+++ b/share/roundup/templates/jinja2/html/issue.item.html	Fri Oct 
23 18:21:40 2020 +0100
@@ -32,7 +32,7 @@
       var node = $('#change_note')[0];
       var initSimpleMde = function () {
         node.parentNode.appendChild($('<input/>', { type: 'hidden', 
name: 'msg-1@type', value: 'text/markdown'})[0]);
-        var simplemde = new SimpleMDE({ element: node, status: false, 
styleSelectedText: false });
+        var simplemde = new SimpleMDE({ element: node, status: false, 
styleSelectedText: false, renderingConfig: { singleLineBreaks: false } 
});
         simplemde.render();
       };
       {% if context.id %}