Two things ... if you installed h5p as a plugin in 3.5, how well does upgrade handle addon plugin -> core?
And think I observed this with 3.9.x ... to keep core small, h5p did acquire the libraries separately ... or maybe not the reason for keeping core small, but getting compat libraries added to Moodle as there are some things h5p can do that haven't reached Moodle yet (present in WordPress, however).
Few plugins work in that fashion me thinks ... addons usually say/warn part of a set and one should get the block etc as well, h5p doesn't ... but that's good? (as long as it works).
'SoS', Ken