The HEVC extensions are only relevant for Window's own media player, so I'm not even sure why you think it has anything to do with this. the rest we can't know. You have not offered any info about the actual source files, i.e. what codecs they use, where they were recorded/ generated and so on. Could well be that they use some outdated QT stuff that is no longer supported in AE or a DirectVideo-CoDec that AE has never supported to begin with or some such thing.