So a bit of an update to this question.
I realize I need to be more polite about calling out bad code. This scenario has come up again and I have found someone's code that has introduced a bug. Note that I have introduced plenty of bugs before and I am not the one to judge. I am just stating that this has happened again and I want to go about things the right way.
This person already doesn't like me and is already sensitive about his code. He is also a senior software engineer while I am just a regular software engineer. How should I do this? Note that we are both working remote. My honest best guess is to just message him. I know he doesn't want to hear or see this but I don't know what else to do. Let me know what you guys think.
Update: I have talked to a higher up and they have told me to go to the person that coded it. This is a person that doesn't like me and is ultra-sensitive about someone telling him his code introduced a bug (even politely). Wish me luck.