here a video about the issue, sliding opacity from 0 to 100% is giving only ON-OFF state to the effect (saw the same issue with 2017 version of Kdenlive) switching regional settings of "numeric separator" from , to . solve the issue, but is not possible to load all projects created using "," issue present on Italian and other languages that uses "," as a separator.
Created attachment 120003 [details] Video showing the issue. from 0 to 100 of slider change the final parameter to anly 2 values: 0-99 = 0 99-100 = 100
Thank you Davy for reporting. I added it as another example to issue: https://invent.kde.org/kde/kdenlive/issues/78 . Hoping JBM or Alcinos can fix that.
another related issue (probably) https://invent.kde.org/kde/kdenlive/issues/333
Closing, seems fixed with latest comma/point fixes.