I don’t know when this changed or why, to be honest, I did not know until now that fields stored in the DB use the field/group ID here rather than the key. My suggest about getting an answer to why the change would be to submit a ticket here https://www.advancedcustomfields.com/contact/
As far as fixing it how, you can use the ID if you use the function acf_get_field($selector)
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