It is as expected and documented:

In scalar context, each execution of m//g finds the next match, returning true if it matches, and false if there is no further match. The position after the last match can be read or set using the pos() function; see pos. A failed match normally resets the search position to the beginning of the string, but you can avoid that by adding the /c modifier (for example, m//gc). Modifying the target string also resets the search position.

The position where to continue matching is linked to the variable that is used, not to the regex!.