See inline comments....

Try this:

A1 = 123abc

B1 = your formula

Insert a new row 1 and see what happens.

ROW($1:$999) becomes ROW($2:$1000)

So this now starts at the 2nd char and the result of the SUMPRODUCT is 1
char less than it should be

MID(A2,ROW($2:$1000),1)

Formula result: 12

I was concentrating so on the second number changing (for rows no where near
the beginning row), that I completely forgot about the first number changing
too. This "repair" should (unless I missed something else<g) do what the OP
wanted and still be non-volatile...

=--MID(A1,MIN(FIND({0,1,2,3,4,5,6,7,8,9},A1&"01234567 89")),SUMPRODUCT(--ISNUMBER(--MID(REPT("
",999-LEN(A1))&A1,ROW($1:$999),1))))

I'm just being especially anal today! I should be ok once I get
some sugar in my system!

Sugar? Not coffee?

Rick