Just a heads up on loose terminology.
A Plugin is a project that solely uses the SpongeAPI code.
A Forge Mod is a project that solely uses the Minecraft Forge API & Minecraft internals.
A Core Mod is a project that changes the code of Minecraft and Forge as the game starts up.
There are Hybrids that can span across, but generally those complicate the terms people use.
As for the blocks appearing, the only cause for this is Mods or Core Mods (Sponge included) potentially messing up the block registry.
In order to help further, apart from giving general advice, people are going to need your Sponge Version, Forge Version, as well as the versions of all your mods.
I’m not a Forge Modding expert, but hopefully someone in the community can offer some ideas after we have your Sponge Version, Forge Version, as well as the versions of all your mods.