New Reminder from selected text macro not working

It works fine, copying the text and running the Shortcut to display the Reminders input pop-up, with the cursor in the input field, but it does not paste into that field. However, if I manually enter Cmd-V it pastes into the field.

I added a “display text” action to see if it's actually working, and it doesn't display, so it must be stalling after running the Shortcut. Does a macro normally stop executing when it executes a shortcut?

No -- but, by default, it does wait for the Shortcut to complete execution before the macro continues. You've a classic deadlock -- KM is waiting for the Shortcut to complete, the Shortcut is waiting for input from KM.

Set the "Execute Shortcut" Action's options to "asynchronously" so your macro continues without waiting, but make sure your "Pause" is long enough for the prompt to be ready for your "Paste" Action.

That fixed it! Thanks for taking the time to explain.