Mouse "Back" button works in Chrome but not Safari

This behavior is potentially tied to how Safari handles input events and mouse button mappings. Safari doesn't always natively respond to generic mouse button 4/5 (which are what Logitech usually maps to "Back" and "Forward") unless they're explicitly remapped or translated into recognized shortcuts. Chrome and DuckDuckGo (as a Chromium-based browser) tend to interpret those mouse buttons more liberally or are better at catching them as browser navigation commands.

Some things you can check/try:

• Using the Logitech Options+ app, select your mouse. See if button 4/5 are mapped as "Back" and "Forward" or are using default functions. Try switching their function to “Keystroke Assignment” and manually assign them `⌘ + [` for back and `⌘ + ]` for forward.
• On your Mac, go to System Settings > Mouse. Look to see if your mouse has a custom profile or settings that can be adjusted.
• With the newer versions of macOS, Accessibility settings could block input for privacy/security. Go to System Settings > Privacy & Security > Accessibility, and ensure Logitech Options has permission to control your mouse.