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Try changing hierarchical: false in your CPT definition (if your application allows). When CPT's are set to hierarchical: true all posts will be queried in the admin dashboard which can cause memory issues.
The problem was that the CPT was set to hierarchical, switching this to hierarchical: false in the functions.php resolved the problem.
This is more about string and array manipulation with php than a WordPress related question. The proper way to do it would be using a repeater field instead of a single text input. Advanced Custom Fields, which seems to be the plugin you are using, has a nice repeater addon. That would let the users add as many fields as they want, change their order, ...
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