Dear CH_Ryan,
Good day!
Thank you for posting to Microsoft Community. We are glad to assist.
I did the test from my side. Yes, I went through the same situation as you. As you understand, only the sender of the email and the user who reacted to the email can see these reactions, other users cannot see.
Sender:
other people (Neither the sender nor react to the email)
This seems to be the expected behavior of the reaction, and at the same time, there are some limitations to this feature that can also affect the invisibility of the reaction if the user does not meet these conditions.
For reference:
Reactions in Microsoft Outlook - Microsoft Support
https://learn-attachment.microsoft.com/api/attachments/ff9f3940-e008-41bb-8ea5-158f7e439a1e?platform=QnA