There are a bunch of icon changes in civi over a couple releases and shoreditch might not be up to date with all of them. There were similar reports for "more" dropdown icons. You could try the latest master version of shoreditch.
In the end, I directly changed the CRM_Case_BAO_Case::getCaseActivity function instead of creating an extension.
Specifically, I changed this function's $where variable to:
$where = "
WHERE cca.case_id= %1
AND ca.is_current_revision = 1
AND cov.grouping = 'visible'";
Here, "cov" is the alias ...