My thinking is this is probably not the reason.
I would think the 928x can happily decode multiple HEVC streams, and would need to be able to do this to support picture-in-picture etc.
The semi-red flag is I have never seen mention in any specs the newer SoC support P7, they just list supporting P5 and P8, but not P7.
P5 and a less extent P8, is the direction for DV for the streaming generation, P5 is close to matching the Colour Depth of P7 FEL, only limitation is no one supports high-bit rate content that is close to the rates as encoded on 4K BluRay.
So it seems likely P7 was dropped out of the SoC support in the silicon in the later iterations - maybe the composer element or something else (speculation).
And noting P7 is specifically for BluRay so that would also make sense as the silicon is not designed for BluRay players, but TV boxes, something that may have been a bit more ambiguous previously.
Can still hold out hope it may yet be there and the above is not the right thinking, it needs analysis on what is happening when attempting to play P7 FEL on these devices.
Also with enough time and effort - given it very likely the SoC can decode two HEVC streams easily (not least because the EL is just a 1080p stream), if there is enough HP in the CPU then could implement an open source composer solution like DoViBaker given time and effort.