It may be a Configurate issue. Configurate is the library to use for configs in Sponge. A new version of Configurate(3.2) is used since API 5, commit 0145f2cabdd13289cff0440bda0c9568deec285e.
SpongeForge uses it since commit 65019f76a78d243d8d80ff64d94a09855a02fe40.
SpongeVanilla since commit 42038102d5ee37c3e7541abcd7cf5405ba3f111a.
Use a build before that commit; SpongeVanilla build 1.10.2-5.0.0-BETA-137 or before; SpongeForge: I think there is no easy way to find it out, you would have to download the SpongeForge builds and compare their commits with the commit mentioned above.
tl;dr
Just try an older build of Sponge until I have figured out if it is a Configurate bug.