Json in entity have backslashes

I add an array to the property, which is converted to JSON.

In the original, the caster produces a good result, but when saving to an entity (not yet saved to the database), backslashes appear in output:

PHP Code:

//  is wrong string (json-array)
$entity->attributes['tables'] string (29) ""[\"one\",\"two\",\"three\"]""

// cast after normally
$entity->tables string (21) "["one","two","three"]"

// raw casting array
json_encode(...) string (21) "["one","two","three"]" 


where problem?

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