Thank you so much for your quick response. I did rerun the whole program and still had subscripts. However, I then updated the Physics package and now it works OK.
I had just purchased Maple 2020 last week (specifically to run your tutorial) and, since, during the installation, it said it was installing updates, I assumed I had the latest version. Sorry for troubling you.
I would like to thank you for making the effort for creating this tutorial. It is just the kind of information that I have been looking for. For example, I had not previously figured out how to just use 3D spatial dimensions (no time).
I am going to presumptuously make a minor suggestion. For me (and I assume the typical user of this tutorial), it would be helpful if you did not make such an effort to get concise code (eg, one line), and instead used a few more lines. I find that the tricks you use in this code are confusing and not helpful for learning the basics of the Physic package. However, this is a very minor complaint, and, please, provide more of these Physics tutorials if you have the time.