Black and white conversion changes to a brown and white?

EDIT oops, Jao's post was "below the horizon" when I posted... 😉  Yes, this definitely sounds like a bad profile.

Please post a screenshot of the entire Lr Develop UI (the full screen).

This sounds suspiciously like a defective monitor profile. Not all parts of the Lightroom UI are color managed, but the histogram is. If the neutral gray there also has the brownish color, that confirms the profile is bad.

Brown looking black and white is also possibly a result of a corrupt monitor profile. These are common on windows machines and affect Lightroom different than photoshop. Typically if this is your problm, also the histogram display looks brown instead of neutral grey. If this is the case, try recalibrating your display with your calibration device. If you don't own one, try going into the monitor's properties and remove any profile you see associated with the display in color management. 

After conversion, what are the some of the RGB values shown (or optionally Lab values) below the Histogram? 

Lightroom Classic version: 10.0 [ 202010011851-ef6045e0 ]
License: Creative Cloud
Language setting: en-US
Operating system: Mac OS 10
Version: 10.15.7 [19H15]
Application architecture: x64
Logical processor count: 12
Processor speed: 2.6 GHz
SqLite Version: 3.30.1
Built-in memory: 16,384.0 MB
Real memory available to Lightroom: 16,384.0 MB
Real memory used by Lightroom: 7,173.3 MB (43.7%)
Virtual memory used by Lightroom: 21,833.1 MB
Memory cache size: 1,097.7MB
Internal Camera Raw version: 13.0 [ 610 ]
Maximum thread count used by Camera Raw: 5
Camera Raw SIMD optimization: SSE2,AVX,AVX2
Camera Raw virtual memory: 2436MB / 8191MB (29%)
Camera Raw real memory: 2783MB / 16384MB (16%)
Displays: 1) 3360x2100

Graphics Processor Info:
Metal: AMD Radeon Pro 560X

Application folder: /Applications/Adobe Lightroom Classic
Library Path: /Users/joyceperlman1/Pictures/Lightroom/Lightroom Catalog-2 (original)-v10.lrcat
Settings Folder: /Users/joyceperlman1/Library/Application Support/Adobe/Lightroom

Installed Plugins:
1) AdobeStock
2) Aperture/iPhoto Importer Plug-in
3) Facebook
4) Flickr
5) Nikon Tether Plugin

Config.lua flags: None

AudioDeviceIOBlockSize: 512
AudioDeviceName: $$$/dvaaudiodevice/SystemDefaultAndEffectiveDeviceName=System Default (MacBook Pro Speakers)#{comment}DVAAU-4201250: Open the audio hardware preferences page.
AudioDeviceNumberOfChannels: 2
AudioDeviceSampleRate: 48000
Build: LR5x42
CoreImage: true
GL_ACCUM_ALPHA_BITS: 0
GL_ACCUM_BLUE_BITS: 0
GL_ACCUM_GREEN_BITS: 0
GL_ACCUM_RED_BITS: 0
GL_ALPHA_BITS: 8
GL_BLUE_BITS: 8
GL_DEPTH_BITS: 24
GL_GREEN_BITS: 8
GL_MAX_3D_TEXTURE_SIZE: 16384
GL_MAX_TEXTURE_SIZE: 16384
GL_MAX_TEXTURE_UNITS: 8
GL_MAX_VIEWPORT_DIMS: 16384,16384
GL_RED_BITS: 8
GL_RENDERER: AMD Radeon Pro 560X OpenGL Engine
GL_SHADING_LANGUAGE_VERSION: 1.20
GL_STENCIL_BITS: 8
GL_VENDOR: ATI Technologies Inc.
GL_VERSION: 2.1 ATI-3.10.18
OGLEnabled: true
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4. No odd Preset?

3. Are you doing anything odd to Temp or Tonality?

2. Just in case, what Process Version is in use?

Some inquires

1. What Profile are you applying?

Please post your System Information as Lightroom Classic (LrC) reports it. In LrC click on Help, then System Info, then Copy. Paste that information into a reply. Please present all information from first line down to and including Plug-in Info. Info after Plug-in info can be cut as that is just so much dead space to us non-Techs.