ANZAC or Anzac?

[Grant Bowker]

My perception, formed by Canadian military writing guidelines, is that at least military acronyms (and most, if not all, others) are fully capitalized.
Perhaps the fact that ANZAC can be pronounced as a word, unlike CMP, MCP, RCAF, RCN, RSL, RCL, WRT, WTF has hastened the conversion. If this is the case, acronyms such as CANSOFCOM will be up for conversion shortly.
Another explanation is that spellcheckers can be set to accept initial capitalization for proper nouns but most require acronyms to be either entered one at a time or accepted as valid on review. (Though this wouldn't explain the 1915 usage....)