So apparently[1] it shouldn't be a separate geometry after all. Flexipost lanes get mapped as

> cycleway:separation=flex_post

along with

> lanes:bicycle:forward=1  
> lanes:bicycle:backward=1  
> lanes=<car lanes + 2>

I'll check and amend when I do the rest of this cluster.

ref:
1: https://wiki.openstreetmap.org/wiki/Key:cycleway:separation
2: https://wiki.openstreetmap.org/wiki/Key:lanes