Yes. It is entirely appropriate, and considered excellent form, to include action and narrative and character reactions in paragraphs starting with that character's dialogue.
This is practice is entirely consistent with the principle of "one speaker, one paragraph."
This is because your action beat and narrative all focus on that character. If, on the other hand, your narrative focused on the actions of another character in the scene, then that shift in focus might warrant a new paragraph.
I am not talking about a change in POV, but a change in where the narrator has their focus. Paragraphs are a way to signal that the POV character's or the narrator's attention is shifting to something else that the author thinks is important to emphasize.
For example, if your example read
“An isotope is determined by the number of neutrons in an atom," said Jane, reading aloud, from the back of the cereal box. John shrugged and twitched as the cesium-127 lacing his morning bowl of fruit loops destroyed his nervous system from within.
That shift to John might deserve its own paragraph since the dialogue and the action are about two different characters -- assuming the Jane is the POV of the scene. It's like the narrator turned their head from Jane to look at John dying. A way to think about it is like a film or video. If the camera of your POV character needs to pan to take the moment in, then a new paragraph might be appropriate.
The argument for when that is not necessarily accurate is when text relates the POV character's reactions to events or actions taken by characters. For example, the paragraph described Jane's smile as John died of poisoned fruit loops, then it would be more appropriate to be part of the same paragraph as the dialogue -- or direct thought as in your example.