Before you comment gets lost in comments. Personally im not sure whats going on. However I will tell you this.
Sponge does not revert mods to previous versions.
I would create a dedicated thread for your issue (and provide a log to your server as that helps work out whats going on) as comments to a thread (such as this) are supposed to be related. In this case related to the download issue.
As for my theory on whats going on based on your description is that the fix the mod developer has done uses somewhat dangerous code, such as async whereby the sponge cause tracker (that tracks everything happening on the server) couldnt work out what to make of it so it either cancelled the RLCraft fix (not sure if sponge does that) or the tracker changed a value, resulting in RLCraft to crash (a log would confirm that)