Had a similar experience three years ago, flying Business Class with my wife (both Gold members) from BKK to Italy. The ticket included a segment with Lufthansa from FRA to Italy. Due to a change of timing the ticket had to be reissued, but Qatar “forgot” to re-issue the ticket for my wife so the segment was cancelled (obviously they did not admit this, found out by calling Lufthansa directly).
It was not the error which I found unacceptable, but the lies and idiocy of the representatives: being told that my wife’s flight is cancelled, but mine is not (we were on the same flight), being told it’s best we contact Doha directly because they can only send emails to the main office and it takes them a while to reply, not trying to solve the issue at all but refusing any solution I offered, even being subsequently accused of no-show/incorrect documents (luckily we had a print-out from the agent with the flight cancellation/denied boarding). Only once we involved the authorities that somebody from Qatar came forward to us and the issue was finally solved (it took almost a week).
Their onboard product might be world leading, but I have not step foot on a QR aircraft since, and have no intentions to do anytime soon. So far I have been pretty happy with the competition!