Hi, thanks for the bug report. I have lowered the severity of the bug, because it would mean that denemo can not be released with Debian.
I think the data loss is not severe enough, as it only happens in a specific situation, and only if you have not saved your session. Anyhow, I understand that this is an annoying bug, I am trying to fix it, but no promises as I am not the author of the package. I have forwarded your bug report to the denemo authors, no reply so far. Günter