As the title say, the dragon fella disappeared from the area he was supposed to be in. The dialogue disclosed as normal, and as my decision was to release him, it made no real difference. But this issue was not that before, in my other two plays. I guess it started with the last patch of yours... but it is just a guess. The save in the dragon's sanctum follows:

https://drive.google.com/file/d/1oRdWBvW8_pfwSSjGlNYpCVcPuA0XWqW1/view?usp=sharing

Salutations Brunachos, 

 

Invisible dragon you say? That is certainly is going into a report for investigation. By any chance could you also send an output log for when you enter the prison? Thank you as always for enjoying the Deadfire.