We bought a Woodland Explorer Map from a villager and travelled 10k blocks in survival mode, but we couln't find a mansion there...
I created a new world in sinleplayer with same seed and the mansion generated.
Seed is -815032661
Environment
Minecraft 1.13
Java 1.8
Used Vanilla Server on Debian 9
@unknown: This bug report has been resolved, no need to keep updating it.