Using New Outlook on Windows for personal email, calendar, and contact management
In this case, there is not a good selective fix in New Outlook. The problem is that New Outlook’s Editor sees the final period in abbreviations such as a.m. and p.m. as the end of a sentence.
Microsoft currently gives New Outlook only the general Auto capitalization toggle; it does not expose the AutoCorrect > Exceptions > First Letter list that Classic Outlook has.
For now, the easiest workaround while keeping Auto capitalization enabled would be to use “8 AM to 9 PM” instead of “8 a.m. to 9 p.m.” Unfortunately, I don't see a way in New Outlook to tell Editor specifically not to capitalize after “a.m.” or “p.m.” Or another potential workaround, when it changes to to To, press Ctrl+Z immediately rather than Backspace. That normally reverses the automatic correction without deleting what you typed. I tested that and it worked for me.