Open it, save as IDML, reopen and see what happens...
I did try that, spaces returned, but kerning was still set for Metrics - causes reflow.
Fixing was the easy part, finding a reason why it happened is what I'm after.
The issue of the missing Optical Kerning when exported as IDML has been noticed before; see, f.e., Missing elements in INDD to IDML conversion
Only thing you can do is report this to Adobe as a bug: Feature Request/Bug Report Form – and hope your vote finally is enough to tip the scales into them fixing it.
That's not exactly what happened.
I opened "their" Indesign 2014 document, not the IDML, in Indesign 2017.
And I can not reproduce the error.