Chrome Version: 70.0.3538.110
OS: Linux, Mac OS 10.14.1, Win10

What steps will reproduce the problem?
(1) Navigate to URL http://watir.com/examples/nested_tables.html
(2) Open "Developer tools" pane, select Console tab, and run the following JavaScript code to create two variables contain XPaths:
xpath1 = "(.//*[local-name()='table'][starts-with(@id, 'tbl')])[12]/*[local-name()='tr']"
xpath2 = "(.//*[local-name()='table'][starts-with(@id, 'tbl')])[12]/*[local-name()='tbody']/*[local-name()='tr']"
(3) Run the following JavaScript code to evaluate an XPath expression containing a union:
document.evaluate(xpath1 + '|' + xpath2, document.documentElement).iterateNext()

What is the expected result?
The XPath should return one <tr> node, since xpath1 returns 0 nodes while xpath2 returns 1 node.

What happens instead?
The XPath returns no nodes, and the JavaScript code in step 3 results in null

Note that the following expression (which switches the order of xpath1 and xpath2) evaluates correctly:
document.evaluate(xpath2 + '|' + xpath1, document.documentElement).iterateNext()

This issue appears to be related to the use of parentheses. If xpath1 is modified to remove the pair of parentheses (which isn't really necessary), then the JavaScript code in step 3 works properly.