Still don't know why I couldn't get it to realize it had hit end of string

It's seen as somewhat of a bug. It happens when the search expression can match a zero length string. In the followig, the parens indicate what the three passes of your earlier s/// matches:

(a@)(b)()
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Or in terms of @- and @+:

pass	@-	@+
1	0	2
2	2	3
3	3	3

The /g loop ends when the fourth pass matches the same thing as the third pass and pos+1 is beyond the end of the string.