Bear in mind, this is a completely amateur opinion.
Maybe your original MRI was unclear and the neurologist decided they needed to see one with contrast dye? This means they can see more definition, given the additive of a dye. The LP isn’t a necessary part of an MS diagnosis, wheras lesion(s) on MRI are. So regardless of whether your LP was positive or negative for Oligoclonal bands, perhaps there just wasn’t the evidence for either an MS diagnosis.
Have a look at https://www.mstrust.org.uk/a-z/magnetic-resonance-imaging-mri where they write about Gadolinium dye.
I am assuming that in your original MRI they didn’t pull you out midway through and inject contrast dye into a vein (usually through a cannula)?
It could be that the scanned images weren’t completely clear that you had lesions in the brain? Or that maybe your LP was negative and that your scan wasn’t absolutely clear?
An LP which is positive for Oligoclonal bands isn’t a definitive pointer for an MS diagnosis as some people do not have O bands and yet they are diagnosed with MS. But the majority of people do have O bands in their CSF. See https://www.mstrust.org.uk/a-z/lumbar-puncture
At least you don’t have long to wait for your neurologists appointment.