Attributes of source: 'meta' not available/undefined in save on pageload

[ckanitz]

Describe the bug
I have a metavalue with key mymetakey.
My blockattributes:

const blockAttributes = {
  ...
  metaAttribute: {
    type: 'string',
    source: 'meta',
    meta: 'metakey',
  }
}

Within my edit function I modify the attribute with onChange={ ( value ) => setAttribute( { metaAttribute: value } )

In my save function I get the attribute as usual with const { ..., metaAttribute } = attributes;

Problem: If I reopen / reload the post at the backend metaAttribute is initially undefined. That leads to either a "whitescreen"-error with length of undefined because, well, the attribute itself is undefined or a blockvalidation-error if I handle it with a default value.

Just a moment later the save function is triggered again and console.log( metaAttribute ) got the correct value. But that doesn't help with the problem since blockvalidation is triggered before the attribute is read from post meta.

To Reproduce
Steps to reproduce the behavior:

1. Have an attribute from meta
2. setAttribute within edit function
3. use the metaattribute within save

Expected behavior
No errors ¯_(ツ)_/¯
The attribute should be the saved value and not undefined on "init"

Screenshots

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Additional context
This issue might be duplicated, see: #4989
If so please let me now so I'll follow the other issue instead.

[aduth]

Thanks for the report. I do think this a duplicate of #4989, but the other issue could have done better to explain more detail to how the bug occurs. I've tried to elaborate on this point at #4989 (comment).