Ive been looking into this as well. it seems that the new firmware update for the A7IV (which adds lossless compressed) is not yet supported by lightroom. I think its specifically Lossless compressed M(edium) . I think we will have to wait for an update to lightroom
When does this happen? And just so we know how to help - which version of Lightroom and which operating system are you using? This information will help one of us figure out your problem.