To expand on wigy's answer, basically the bitvec type and lsb/msb identifier were reversed. To put this into context, instead of the metadata types returning
BitVec<u8, lsb> it returned
From a middleware perspective, e.g. the Python, Java, JS, Go, ..., etc. APIs this could be quite problematic especially in the cases where they assumed a fixed ordering for the type fields. Since we also don't get the version of scale-info in the metadata to allow for different codepaths, this was an unintended breaking change and could have required upgrades for all developers building cli tools, UIs, etc. on top of Substrate.
The JS API was certainly impacted immediately when the original 3.0 was used in Substrate, although it is now a bit smarter and can handle these fields in any order by looking at the type paths to detect which field goes where.