I haven't found this behaviour, and I suspect this may be some confusion over what alt text and title text do.

In internet explorer alt text used to be shown as a tooltip on hover, but this is not what alt text was intended for, it is only meant to be shown when the image doesn't load or if the user is using a text only browser/screenreader. modern browsers show title text as a tooltip instead.

If I look at the source generated by the first hot graphic in the default adapt course, the following JSON:

"_graphic": {
"src": "course/en/images/hotgraphic.png",
"alt": "alt text",
"title": "title"
},

produces the following HTML:

<img src="course/en/images/hotgraphic.png" alt="alt text" title="title">