When applying "Speed Effect", the negative options doesn't slow the video, it applies the same treatment as the positive option. - 150% = 150%
What happens if you use 50%?
Can you test with latest AppImage?
The speed effect was completely re-written. Your issue is fixed as well. Feel free to reopen this bug or create a new report if this is still happening with the latest version (https://kdenlive.org/en/download/)