There was a bug in 4.13.0 that would allow this to happen. I think this was fixed in 4.13.1 or it may be 4.14, I can’t remember exactly.
You’re not the only one who has fallen victim to this Build Setting. Most who don’t actively use or worry about a lot of the lightmapping intricacies or have custom lightmaps really know or worry about this, so definitely not anything to be embarrassed about. 