Impossible to recover after crash

lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 20.10
Release: 20.10
Codename: groovy

Shotcut version 21.05.01

I was working on an MKV-file for some hours using just one track and the file has the resolution of 1280 * 720 px. The other time I triggered undo the application froze. I waited many minutes and returned to the computer for discovering that the application was closed. I reopened it. I clicked the project name I was editing before the crash and chose to recover the project. The first frame of the file appeared however the timeline and playlist were empty. I opened MLT-file in emacs and saw that it missed all the huge amount of changes I had made to the project. This time, I killed the application and reopened it following by opening the same project. Instead of the first frame, INVALID was displayed. I also had a look at the recovery folder and it contained nothing useful. Shotcut just destroyed everything I did during these hours. This isn't acceptable. In addition to this case, I have three other projects that are impossible to reopen. This all can render Shotcut useless as similar things happened to Kdenlive that led me to stop using it. I went over to Blender before I discovered Shotcut. I've made many successful projects with Shotcut however these four unsuccessful projects describe the application as not trustworthy. An app should not destroy anything however Shotcut does sometimes. You can blame melt or something else, I don't know, for me as an end user, Shotcut is responsible for the fail of these four projects. I also add the current MLT-file that has not much useful information in it anymore and the application log if that can help you tracking down the reason for these deletions.
crash.txt

matemaatika-daliaga-20210310-nurgavõrrandid.zip

Start by examining the attached crash.txt, the damaged MLT file, and the recovery-folder contents, then investigate project recovery after a crash in Shotcut 21.05.01 on Ubuntu 20.10. Done means a recovered project retains its timeline and playlist rather than opening empty or showing INVALID.