Yes, it’s only with S912 SoC.
On many boxes it can be avoided with a line in “advancedsettings.xml”
This is my advancedsettings.xml:
<advancedsettings>
<gui>
<algorithmdirtyregions>0</algorithmdirtyregions>
</gui>
</advancedsettings>
When placed in “storage/.kodi/userdata” folder (and reboot) it solves subtitle stutter on my S912 box