prefix table and plugins

Short answer: Just a badly written plugin.

Longer answer: When a plugin calls the database in the right/correct/best practice way it uses Global $wpdb instead of just naming the database table, for example:

Wrong way

$sql = "SELECT * FROM wp_posts WHERE... "

Right way

$sql = "SELECT * FROM $wpdb->posts WHERE... "

this way it would work even if you change your tables prefix.

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