Adding +00:00 to the end of your date string is creating a UTC offset. Since you're specifying zero hours, zero minutes, and zero seconds, it's creating a zero-offset from GMT, or in other-words, setting your timezone to exactly GMT.

For example, I'm in a GMT -5 timezone (US Central). When I create a new JS date using today's date at 6am, I get a subtraction of 5 hours (because GMT -5 is my timezone) :

> new Date("2022-06-08T06:00:00+00:00")
// Wed Jun 08 2022 01:00:00 GMT-0500 (Central Daylight Time)

However, if I omit the offset, then it works as expected, and I get the date in my local timezone.

> new Date("2022-06-08T06:00:00")
// Wed Jun 08 2022 06:00:00 GMT-0500 (Central Daylight Time)

I can also run your code examples and get the expected result.

> new Date("1969-07-26T03:00:00")
// Sat Jul 26 1969 03:00:00 GMT-0500 (Central Daylight Time)

See the section Date Time String Format in the ECMAScript specification for more details.

I believe the reason is that from Sun, 6 Apr, 1969 00:00 to Sun, 5 Oct 1969, 00:00 the America/Buenos_Aires timezone observed a UTC offset of UTC-4h. So I don't think this is an error, rather it is reflecting what the UTC offsets were at this time.

Before 1 Oct, 1963 00:00 (from ~1946), the UTC offset was always 3 hours with no DST.

So the UTC offset in this timezone has varied over time, from UTC-2 to UTC-4, depending on the year and the date. Since Sun, 15 Mar, 2009, the UTC offset has been UTC-3.

I'll add more examples of the varying UTC offsets below, two from your original example and some more to illustrate the varying UTC offsets.

const timeZone = 'America/Buenos_Aires';
const fmt1 = new Intl.DateTimeFormat('en-GB', { timeStyle: "medium",
  dateStyle: "medium", timeZone: 'UTC'});
const fmt2 = new Intl.DateTimeFormat('en-GB', { timeStyle: "long",
  dateStyle: "medium", timeZone});

const examples = [
    new Date("1962-07-26T03:00:00+00:00"),
    new Date("1963-07-26T03:00:00+00:00"),
    new Date("1964-07-26T03:00:00+00:00"),
    new Date("1969-07-26T03:00:00+00:00"),
    new Date("2008-10-19T03:00:00+00:00"),
    new Date("2010-07-26T03:00:00+00:00"),
    new Date("2020-07-26T03:00:00+00:00"), 
];

console.log('UTC'.padEnd(25, ' '), 'Local time');  
for(let dt of examples) { 
    console.log(fmt1.format(dt).padEnd(25, ' '), fmt2.format(dt));
}
 

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Here is a list of the UTC offset changes for the timezone from 1950 onwards, this illustrates the fact that is has changed many times. until 2009 when it has remained at UTC-3.

let { IANAZone, DateTime } = luxon;
const timeZone = new IANAZone('America/Buenos_Aires');

let date = DateTime.fromISO('1950-01-01T03:00:00Z').setZone('UTC');
let endDate = DateTime.now();
let lastOffset = 0;

function formatUTCOffset(offsetMinutes) {
    return (offsetMinutes / 60) + ':' + String(offsetMinutes % 60).padStart(2, '0');
}

console.log('Time'.padEnd(24, ' '), '\t', 'UTC Offset')
while (date < endDate) {
    if (lastOffset !== timeZone.offset(date)) {
        console.log(date.toISO(), '\t', formatUTCOffset(timeZone.offset(date)));
        lastOffset = timeZone.offset(date);
    }
    date = date.plus( { day: 1 } );
}

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