Mouse buttons 4 and 5 sometimes aren't captured properly in controls settings

The bug

Occasionally, mouse buttons 4 and 5 are not captured correctly. In that case they're displayed as "☐\☐]☐", where ☐ is U+0001 (Start of Heading). If this happens, mouse buttons 4 and 5 are assigned to the same action.

I am not sure exactly when this issue occurs. It just happened to me while testing MC-156810.