Awk substr index 0

I just discovered that substr() in awk accepts either 0 or 1 as the initial index in a string. I tested this in Gawk 5.1.0 and MacOS awk 20070501. awk 'BEGIN {print substr("abcd", 0, 1)}' outputs "a", as does awk 'BEGIN {print substr("abcd", 1, 1)}' and awk 'BEGIN {print substr("abcd", 2, 1)}' outputs "b" just to prove that nothing's obviously wrong. I didn't see anything in the man pages or the Gawk info file other than mentions of 1-indexing. For consistency with the documentation and with the fact that index() returns 1 for the first position and 0 for no match, it would be good policy to always use 1. My question is why is this duality the case? Also, is it documented somewhere? Are there other awk implementations that do this?