I want to emphasize that this doesn't exist. It's just one that I thought of.
This is possible and makes sense, so you are dealing with two complete sentences.
There used to be (and maybe still are, I'm not sure) schools of writing teaching that in such a case, you can't join them with a comma; only a semicolon is acceptable.
Truth is, you can use either a comma or a semicolon. However, the two aren't completely freely interchangeable. A semicolon feels formal and "proper", while a comma gives a more casual vibe. You should generally pick the one than fits the style you're writing in, using the other would be jarring. (See what I'm doing here?)
If you don't have two complete sentences that can stand on their own, such as...
Truth is. You can use either a comma or a semicolon.
- Yeah, this doesn't work. "Truth is" is not a complete sentence.
...then it's not that a semicolon isn't needed, it's that you can't join them with a semicolon, you have to use a comma there.