changes don't get marked as merged despite successful merge

the most recent instance is https://codereview.qt-project.org/87974, integrated as d147535322.
the problem might be related to the messages about a locking failure.
i wonder whether this had anything to do with the high cpu usage gerrit had today.

there is another variation: https://codereview.qt-project.org/91138, integrated as 6ccb1f47aa.
that time, gerrit thought it needs to claim it abandoned the change.