Yeah, sorry, didn’t make it into 26.08.0 but will definitely be included in the next maintenance release.
You can fix it temporarily by locating encodingprofiles.rc and editing it in the text editor of your choice. Replace all occurrences of -vsync 1 with fps_mode cfr.
Do the same for kdenlivetranscodingrc (note the missing dot before rc!) and replace -vsync 0 with -fps_mode passthrough.
There is one more change needed but that is in the source code and only affects silence removal.