OnePlus 6, stock, MS Launcher beta 184.108.40.206883.
Set "Dock" type in settings to `Apps`.
Looks like "Number of columns in dock" options is not a point here. You have to put icons in two or more rows. Check that you can slide it up.
Now keep icons in first row only. Try to slide it up (Home is unlocked). You can do it once only.
"Dock" became unresponsible to slide up. Pay attention that other ways like keystroke assignment allows you to show dock.
Try to add icon to 2+ row.
"Dock" became slideable again.
Attached video shows "Dock" configuration and steps-to-reproduce.