Q: What does it mean if the status changes from Awaiting Decision to Submitted?

This is certainly odd. This could be a glitch in the system or an erroneous update by the Associate Editor (AE). An outside chance is that the journal has sent the manuscript either to a sister journal or to a preprints repository (if the journal has one). What this means, respectively, is that the journal believes your paper would be a better fit with a related journal or that they have decided to reject the paper (and therefore host it only on their preprints repository). However, the first outcome is typically decided before peer review, and in either case, the journal usually first communicates their decision to the author(s). Also, the responsibility to send the paper to the sister journal is the author’s rather than the journal’s. Which is why this update is quite strange.

To calm your doubts, you should write to the journal seeking clarification on this update. In case you need help with email templates for doing so, you may find this handbook useful: A practical handbook of templates for communicating with the journal [It’s from our sister brand, R Upskill, a platform offering a variety of learning programs for the various stages of a researcher’s career.]

And if it helps, you may also go through these somewhat similar queries by other researchers:

• What does a change from "Awaiting editorial decision" to "Awaiting reviewer scores" mean?
• What does a change from "awaiting editorial decision" back to "under review" mean?

Hope that helps. And hope the ‘mystery’ is resolved soon. :)