The proposed fix looks good.  
I wonder if it is worthwhile to have separate loops for Lists vs Abstract Lists, as GetElements is more efficient for Lists. It may be that the difference is not worth the complexity. IDK.

Note that there are a number of places where DECACHE/CACHE calls need to be inserted in TBCE. I've done that in tip-636 (main). [tclExecute.c](https://core.tcl-lang.org/tcl/fdiff?v1=b7ee7c30a6f02625&v2=5d734646930c5089)