Fix STLImpl scene update

See b/317014852 for context. I was unfortunately not able to create a
small sample to reproduce, and therefore to come up with a test. I will
return to understanding this more once I'm done with more urgent tasks.

Bug: 317014852
Test: Manual, see Flexiglass steps from chat linked in
 b/317014852#comment1
Flag: N/A
Change-Id: I12498ca3b464bc422478bdb2a01bd1a5885e04b3