Figured it out:
First, I thought maybe my use of symbols (non-alphanumeric characters) was screwing up the password somehow. I removed these from the BIOS password but I still wasn't getting into the UEFI successfully.
Decided to re-read the user manual again (which does not mention UEFI setup password specifically, only "Setup Password"), but more carefully this time.
Ran across this line:
You can use up to 32 characters in your password.
As you type, placeholders appear in the field.
The password assignment is not case-sensitive. To erase a character, pressBackspaceor the left-arrow key. [Italics Mine]
Well this got me thinking. I configured a multi-case password like aB1234cD in the BIOS, and I was attempting to enter the exact password to enter the UEFI, but it was being rejected.
Here's what was happening: because the BIOS was case-insensitive, it was recording my original password of aB1234cD as the all-lowercase ab1234cd and passing that on to the access control of the UEFI boot manager. Apparently the UEFI boot manager is case-sensitive and so it was not accepting my entry of aB1234cD. When I tried ab1234cd, I was able to enter the UEFI boot manager.