Current version in repo is 1.2.3 which is considered the "old stable".  Latest
version of that branch is 1.2.5 which is needed to support new features.

https://github.com/flatpak/flatpak/releases/tag/1.2.5
The main feature is the support for allowing the use of extra_data
for runtimes (as opposed to only for apps) which is used by the new
org.freedesktop.Platform.openh264 extension.

Here is what happens when I update flatpak apps
----------------------------------------------------------------------

> flatpak update 
Looking for updates���


        ID                                         Arch            Branch      
   Op          Remote          Download
 1. [���] org.freedesktop.Platform.openh264          x86_64          19.08       
   i           flathub         < 593.4 kB
 2. [���] com.spotify.Client                         x86_64          stable      
   u           flathub           115.0 MB / 126.4 MB

Warning: org.freedesktop.Platform.openh264 needs a later flatpak version
Changes complete.