Is it just me, or is the text editor of the webmail interface at outlook.office.com really buggy when it comes to setting fonts? I am unable to select a full paragraph, or select just a sentence, and change its font face or size via the popup menu. The size/face I choose from the popup menu's dropdowns are not being applied to the selected text.
I tested the https://outlook.office365.com using Firefox 99.0 (64-bit) without any problem.
To solve this kind of issue I would suggest you try it with another browser like Firefox, chrome or opera. (Sometimes the browser itself is the problem especially for internet explorer).
I've used https://outlook.office365.com/owa/ with Chrome on Windows 10 for several years. Outlook on the web is ride with bugs and is very sluggish, but I haven't encountered any issues when applying fonts to text, which I've done a fair amount of time.