Here's the rub: Fedora, and thus RPMFusion, rpm packaging guidelines require that the "libs" be packaged into a separate *-libs package. The rpm packages you created through the script does not split out the libs into it's own package, but rather just keeps the libs in the main rpm package, probably the avidemux3-core package. You can confirm this by using:
rpm -qpl avidemux3-core-2.6.0_r0.x86_64.rpm | grep .so
This is just one of many complications one runs into when attempting to install rpm packages into Fedora that are not packaged to Fedora's own packaging guidelines. One way to possibly get around your particular situation would be to circument all yum "dependency" requirements by, first, uninstalling any avidemux packages you already have installed from RPMFusion repos, then using rpm with the --nodeps option to install the package(s) you built.
That might also require manually installing some "dep" packages, and also might require you to add a line in your /etc/yum.conf file:
Unless there is some pressing reason to have the latest upstream version of avidemux, it would be a whole lot simpler to just use what's offered now from RPMFusion.