I don’t know how or why it has been working for you in the past, but for quite some time field keys have had to start with “field_”. This is an often overlooked thing and is only mentioned under “Field Settings” most of the way down the page https://www.advancedcustomfields.com/resources/register-fields-via-php/
Early versions of ACF 4 allowed field keys that did not start with “field_” but it was changed shortly by 4.1 or 4.2, I think, that was a long time ago.
And field group keys must start with “group_”
