[MapProxy] Unusual behavior with configuration symbol names and scrambled tiles?

[Bruce Crevensten]

Hi,

I was playing around with some cache configurations and stumbled across a
behavior I don't understand.  Two very similar configuration files,
experimentally using Bing as a known-good tile provider backend, where one
displays correctly and the other has the tiles in a scrambled order, and
the only difference I can see is the names of the symbols used to reference
sections of the configuration (layers, caches, etc).

Working configuration file:
https://gist.github.com/brucecrevensten/a065ea0f49a039475760

Failing configuration file:
https://gist.github.com/brucecrevensten/288817ea067dd118a85c

Screenshot of failure:
https://github.com/ua-snap/mp-config/blob/master/a.png

Is there a sensitivity regarding the names of the symbols being used that
is confusing the configuration, or am I just mis-reading this?

The Github issue below got me going on the right track by starting with the
config file there, but I still don't understand what the important
differences are between the two configurations I've included here.
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