$options = [ 
	'timeout' => 'default',
];

Note there is a space after the opening [.

$options = [ 'timeout' => 'default',
];

Example from https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/ExternalData/+/575982/1/includes/ED_Utils.php.

It seems the sniff detects the [ sequence with the space, continues as if the array is on a single line, and removes the following newline and indention.

What it could do instead: Check if the opening [ is followed by a horizontal whitespace token (spaces, tabs), directly followed by a vertical whitespace token (newlines). If this is the case, remove the first one instead of the second. Detecting "horizontal" vs. "vertical" tokens is easily possible by comparing the 'line' of the current and next token. String comparisons are not needed.