From the strtotime
manual:
Note:
Dates in the m/d/y or d-m-y formats are disambiguated by looking at the separator between the various components: if the separator is a slash (/), then the American m/d/y is assumed; whereas if the separator is a dash (-) or a dot (.), then the European d-m-y format is assumed. If, however, the year is given in a two digit format and the separator is a dash (-), the date string is parsed as y-m-d.
So your date is invalid. You can work around this using date_create_from_format
instead:
$date = date_create_from_format('d/m/y h:i A', '25/05/19 05:59 AM');
echo $date->format('M d, Y h:i A');
Output:
May 25, 2019 05:59 AM
Demo on 3v4l.org