Tiki's syntax allows paragraphs with ordered or unordered lists. For example, the following dummy list items are inside this first paragraph:
Unfortunately, since r28616, parsing such a paragraph generates an HTML P element followed by a distinct HTML UL or OL element. These are rendered as distinct elements, split by as much space as 2 distinct P elements. This can be seen for example in this blog post, stored in Tiki 15 as of 2018-07-31.
Ironically, this problem happens when feature_wiki_paragraph_formatting is enabled, and not when it is disabled. But there is still a problem when feature_wiki_paragraph_formatting is disabled, because as pointed out by Jonny Bradley, the p element cannot contain list elements in HTML 5. This is explicitly documented in the second note of https://www.w3.org/TR/html5/grouping-content.html#the-p-element
As reverting the change would make HTML invalid, the proper solution would be to either hack some CSS which avoids the p element's bottom margin when it is followed by some lists, or to use the div element instead of the p element, as shown in the HTML specification.