Gianluca, thank you for your reply, as well as the explanation of the "interpretation" context. (I still do not understand where the "zztop" came from.)

Do you have a suggestion for a workaround?

Again, I am trying to concatenate together strings, and expressions like plots, and to retain the input order. I am using ToExpression[] since this is all fed back into CellGroup[ExpressionCell[ ... ]].

Thank you.

Can you add a link to the SE post?

I agree more details about the final goal would be on point as this looks like a XY problem. Meanwhile, here's what I can add:

boxes = RowBox[{"c", ToBoxes[Plot[Sin[x], {x, -2, 2}]], "a"}];

boxes is a typeset/box representation of a product: c Graphics[...] a. Where did it came from and what is your plan? If you only want to display it:

RawBoxes @ boxes

If you want to convert it to an expression but keep the input order:

ToExpression[boxes, StandardForm, Hold]   (*HoldForm or whatever you need*)

But if your initial plan was to put c next to the plot which is next to a then why not:

Row[{"c", Plot[Sin[x], {x, -2, 2}], "a"}]

Yes, though

Cell@CellGroupData[{Cell@
    BoxData@ToBoxes@Row[{"c",Plot[Sin[x], {x, -2, 2}], "a"}]
   }]

may be more convenient

One final note: apparently one needs to keep things "low-level" with CellGroupData[], but also, for the groupings to work, it needs to be fed back into NotebookWrite[] and not CellPrint[].