Shortcuts appearing when using Option+arrow in Outlook on Chrome in Mac

luse Outlook on Chrome in my MacBook Pro.

While typing an email, though, if I use Option tarrow left or right (to go back or forth between words), after a few words, the shortcut letters for the menus pop up, and stop what I'm doing. (See image.)

It doesn't happen in any other window on Chrome. So it's not a Chrome thing. It's only when Outlook 365 is loaded.

(Don't know if it happens in other 365 apps, as I only use Outlook, really.)

Anyone experienced that? And, if so, is there a solution?

I tried Outlook 365's setting, but nothing there.

Thanks!