Reference post @4249123 above - of course, this only happens when Branched Path Tracing and Progressive Refinement are used together.

The manual tells us to switch Branched Path Tracing off when using Progressive Refinement, so I guess this can't really be called a bug. For some tasks I'll continue to use both together, but I think this issue can be ignored.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)