Read this guide to save you plenty of time and to fix this rampant issue, no BS just facts from credible sources.
Fix No Horizontal Mouse Movement
Go to this directory:
씨:\Users\Username\Documents\Battlefield 2042\Settings\
Open the file named Prosave_profile.
Now delete every line that begins with GstKeyBinding.infantry.ConceptYaw
Then replace all of those lines with this:
GstKeyBinding.infantry.ConceptYaw.0.axis 9 GstKeyBinding.infantry.ConceptYaw.0.단추 60 GstKeyBinding.infantry.ConceptYaw.0.negate 0 GstKeyBinding.infantry.ConceptYaw.0.유형 1 GstKeyBinding.infantry.ConceptYaw.1.axis 0 GstKeyBinding.infantry.ConceptYaw.1.button 205 GstKeyBinding.infantry.ConceptYaw.1.negate 0 GstKeyBinding.infantry.ConceptYaw.1.type 2 GstKeyBinding.infantry.ConceptYaw.2.axis 0 GstKeyBinding.infantry.ConceptYaw.2.button 203 GstKeyBinding.infantry.ConceptYaw.2.negate 1 GstKeyBinding.infantry.ConceptYaw.2.type 2 GstKeyBinding.infantry.ConceptYaw.3.axis 0 GstKeyBinding.infantry.ConceptYaw.3.button 8 GstKeyBinding.infantry.ConceptYaw.3.negate 0 GstKeyBinding.infantry.ConceptYaw.3.type 0
Launch the game and see if the issue has been fixed on your end.