Hello there,
I'm currently using the UI.X 2 theme by ThemeHouse. I have noticed that the "ticket message action button" (the ellipsis) is not visible there:

It is, however, visible in the default style:

Considering it worked with Tickets v2.8.1 and stopped working with Tickets v2.9.0, I suspect the add-on to be the culprit here and not the style.
I'm currently using the UI.X 2 theme by ThemeHouse. I have noticed that the "ticket message action button" (the ellipsis) is not visible there:

It is, however, visible in the default style:

Considering it worked with Tickets v2.8.1 and stopped working with Tickets v2.9.0, I suspect the add-on to be the culprit here and not the style.