According to an anon on MW.org, Ext:Arrays doesn't work with MediaWiki 1.33.1; see 1 and 2.

$egArraysExpansionEscapeTemplates isn't initialized correctly so that it ends up (somehow) as an array of arrays instead of an associative array. Compare the two print_r outputs. This is likely because it's initialized in the extension.json file with incompatible syntax.

Incorrect: Array ( [0] => Array ( [=] => {{=}} ) [1] => Array ( [|] => {{!}} ) [2] => Array ( [{{] => {{((}} ) [3] => Array ( [}}] => {{))}} ) )

Correct: Array ( [=] => {{=}} [|] => {{!}} [{{] => {{((}} [}}] => {{))}} )