Hello ,
Actually I have the exact same issue for 2 weeks now . This behaviour occurs on both Edge and Chrome , but it's OK on Firefox. Though I should not really use Firefox on my business laptop .
Of course I contacted at first my company IT service but they are not able to find a way to fix it. It seems to be a patch that still need to be deployed (apparently).
I also can tell than last week , the icons came back for 1 day but now they're gone again.
And in 2 weeks I had time to restart my computer many times, updating new browsers versions several time, cleared my cache, cookies, history, .... nothing changed much.
And I dont have any header called "header "External images" in my Outlook settings -> General -> Privacy and data. So I am also confused and looking for an answer.
Regards 