The dvbapi plugin can either use libdvbcsa or FFDecsa, but the latter is x86/x64 only, so under arm it has to use libdvbcsa. In any case, under x86, I compiled the plugin with libdvbcsa, to avoid any difference in behaviuor.
I find it strange that with the libreelec build system ldd doesn't show a dependency on libdvbcsa.
Oh, I had to patch its Makefile to unconditionally use it (just passing LIBDVBCSA=1 wasn't enough: it was still trying to compile FFDecsa).
Another strange thing is that I had to modify the PKG_SHA256 of many, but not all, vdr plugins from master, as well as several other modification to the patches and/or package.mk, but maybe that's because of the older toolchain in 8.2.x (which could also be the cause of the plugin failing, it's not the first time that a different gcc produces non working code).