Steps to reproduce the issue

Set up a multilingual site. During installation, install localized content.

1. Paste this snippet somewhere:

// Get Category (en-gb)
$category = JCategories::getInstance('Content')->get(8, true);

var_dump(
	// Should be NULL
	$category->getSibling(true)->title,
	// Should be Uncategorised
	$category->getSibling(false)->title
);

2. Paste this snippet:

// Get Category (en-gb)
$category = JCategories::getInstance('Content')->get(8);

var_dump(
	// Should be NULL
	$category->getSibling(true)->title,
	// Should be Uncategorised
	$category->getSibling(false)->title
);

Expected result

1 and 2) Uncategorised should be left sibling:

NULL string(13) "Uncategorised"

Actual result

1. Category (en-gb) is its own sibling:

NULL string(16) "Category (en-gb)"

2. No siblings at all:

NULL NULL

System information (as much as possible)

3.9 nightly, PHP 7.3.8