Created attachment 516352 [details] build log kdenlive used to fail with the above error, but after upgrading to =media-libs/mlt-6.4.1-r5 it can now build.
Created attachment 516354 [details] emerge --info
Yes, unfortunately further mlt stabilisation depends on another bug. This only happens though if you for some reason mix ~arch with a stable version of media-libs/mlt.
Newer mlt is stable now.