isValid fails to recognise valid formats

[rijnhard]

Describe the bug
I am using moment to test if a user-provided format is correct.

moment(moment().format(format)).isValid();

And this always emits a deprecation warning if the format is not correct which is unnecessary.

relates heavily to #2535
and https://stackoverflow.com/questions/44595835/validate-date-format-not-date-string-using-momentjs

To Reproduce

function test(format) {
    return moment(moment().format(format)).isValid();
}

test('YYYY'); // returns true, no warning
test('hamster'); // returns false, and issues warning

Expected behavior
return false without warning.

Screenshots
If applicable, add screenshots to help explain your problem.

Moment-specific environment

• The time zone setting of the machine the code is running on: GMT+2
• The time and date at which the code was run: 20210505T14h25

Please run the following code in your environment and include the output:

Wed May 05 2021 14:28:19 GMT+0200 (South Africa Standard Time)
2021/05/05, 14:28:19
-120
// errors (nodejs not browser)
2.29.1

[rijnhard]

Ontop of this I get some pretty strange behaviour in terms of false negatives with isValid

test('YYYYMMDD'); // true, no error
test('YYYYMMDDHH'); // false, with deprecation warning
test('YYYYMMDD HH'); // true, no error
test('YYYYMMDD HHmm'); // true, no error
test('YYYYMMDD HH mm'); // false, with deprecation warning
test('YYYYMMDD [T]'); // false, with deprecation warning

the inconsistency is unreal, my actual test case is YYYYMMDD[T]HH[h]mm which it utterly fails on.

[ChechenWiki]

Moment.js doesn't have a Chechen (ce) time format support and falls back to Russian, so Notifications are in an incorrect time format. Can this be fixed here?