Same problem here. Device is Tanix TX3 Mini - H. Libreelec boots nicely from sd card with gxl_p281_2g or gxl_p212_2g but installtointernal doesnt work.
Seems like Libreelec doesnt see eMMc on these boxes cause reboot to internal doesn't work either. From what I've read online these boxes (L and H) have different motherboard (v1.3 printed on them, old ones are v1.1). I couldn't install any custom s905w ROM, only the original one from Tanix web site. Does this mean that this boxes need different dtb.img file? If so, how can i get one?