It looks like a bug, that depends on the timezone and hour.
With UTC timezone, it is OK :
$date1 = new DateTime("2022-03-01 00:00", new DateTimeZone("UTC"));
$date2 = new DateTime("2022-04-01 00:00", new DateTimeZone("UTC"));
difference 0 years, 1 months, 0 days
With Europe/Berlin timezone at midnight, we have the 3 days offset :
$date1 = new DateTime("2022-03-01 00:00", new DateTimeZone("Europe/Berlin"));
$date2 = new DateTime("2022-04-01 00:00", new DateTimeZone("Europe/Berlin"));
difference 0 years, 1 months, 3 days
At 01:00 in the morning, it is kind of OK : 0 month but 31 days
$date1 = new DateTime("2022-03-01 01:00", new DateTimeZone("Europe/Berlin"));
$date2 = new DateTime("2022-04-01 01:00", new DateTimeZone("Europe/Berlin"));
0 years, 0 months, 31 days
At 12:00, everything is right again: 1 month
$date1 = new DateTime("2022-03-01 12:00", new DateTimeZone("Europe/Berlin"));
$date2 = new DateTime("2022-04-01 12:00", new DateTimeZone("Europe/Berlin"));
0 years, 1 months, 0 days