This is rsync we are talking about. A bug in rsync basically means lost data and/or unreliable backups.
I think it's normal to be pissed at lost data. Maybe it's not socially acceptable to spit in the face of a volunteer but it's 100% human to feel annoyed by an obvious drop in code quality.
> Maybe it's not socially acceptable to spit in the face of a volunteer
Why are you hedging this? Do you think maybe it is socially acceptable?
The thing is, showing the annoyance to the volunteer, who is already doing their best, has two possible outcomes:
1) they stop volunteering
2) they will ignore you
In neither of that is your issue solved. So maybe it's better to deal with the frustration on your own and then file a bug report.