Never mind, as apparently it's treated as an RPN operator in absents of a new value explicitly being entered, so if a digit is entered and then deleted, it's applied to the new entry, not the stack. (I guess I'd prefer it always apply to the entry value, with some other method used to negate the TOS.)
There's a one-keystroke workaround for what you want. Just press the space key first, before keying any digits. That will enable you to enter negative numbers by pressing [+/-] before the digits. Cool, huh?
The default behavior of pressing [+/-] only after the digits is consistent with many decades of HP calculator tradition.
Disclaimer: I don't work for HP. I've just been enjoying HP calculators since the 1970's.