I'm not aware of any limit. Do you happen to have at least a screenshot of this?
I have run into a limit with a long tooltip that was mostly typed in (I was trying for a "hover for help" effect). I don't know your data source, if you're using Excel, Access, or a text file there is a limit of 255 characters for a text field.
I just ran into this on 8.0.4. I pasted formatted text into my tooltip and it was automatically truncated. I put it into Word to get a character and word count. See the image. Maybe there was a decision made that a best practice or for better usability, a tooltip should not exceed a certain length? To make sure there were no escape characters in my formatted text, I pasted plain text into the tooltip. The Plain text was also truncated.