Steps to reproduce the issue

Using paginated list of items, select 2nd or subsequent page and then select first page

Expected result

Page displayed should show as per selection

Actual result

Return to first page has no effect.

System information (as much as possible)

Shows up in site running the crmery component. On a 'company' page with enough contacts to cause pagination to be invoked.

Additional comments

Problem is caused by test in /libraries/src/Router/SiteRouter.php in Joomla 3.8.1 and /libraries/cms/router/site.php previous versions. in function processParseRules the following test (line 600 in the 3.8.1 version):
if ($start = $uri->getVar('start'))
will ignore the start value if it is zero. Replacing with:
if (($start = $uri->getVar('start')) !== null)
Allows zero as a valid value and fixes the problem.