I've seen this happening when you control multiple duchies and change your primary title, but in this instance I owned just a single duchy of Cornwall and its two de jure counties:
Steps to recreate:
Start in 769 as the Petty King of Cornwall.
Change succession law to Tanistry (not sure if this matters, but it's what I did).
Choose great-grandson as heir (not sure if this matters, but it's what I did).
Die.
The original king was always referred to as a king. The great grandson heir became known as a duke despite the realm continuing to be known as a petty kingdom.
Steps to recreate:
Start in 769 as the Petty King of Cornwall.
Change succession law to Tanistry (not sure if this matters, but it's what I did).
Choose great-grandson as heir (not sure if this matters, but it's what I did).
Die.
The original king was always referred to as a king. The great grandson heir became known as a duke despite the realm continuing to be known as a petty kingdom.
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