I was able to figure out the random tests were failing because I was destructively modifying the given strings. I duplicated them at the beginning and it solved the problem.

How do you explain that 174 guys passed the Ruby kata and 1832 in all languages? Algorithms are the same in all languages.

Sorting was Hard, took my time to figure out what type of sorting was required.
Also achieved to solve it for "N" strings instead of 2 only.

RUBY

I agree that the sorting is unclear in the instructions, but I was able to figure it out after trial and error.

However, I am PRETTY sure the expected answers to the mix random tests are different than how the instructions state they should be.

String1:
Chjha9ftobRkhas@omzx3auefJnvgd
The f count in string1 is 2

String2:
Srmrv;xobs-cgbnWgpifRxhef,uxfl
The f count in string2 is 3

Expected: "1:fffff/1:hhhh/1:xxxx/1:aaa/1:bbb/1:ggg/1:ooo/1:ee/1:mm/1:nn/1:ss/1:uu/1:vv/=:rr",
instead got: "1:aaa/1:hhh/2:fff/2:xxx/1:oo/2:bb/2:gg/2:rr"

The random test expects string 1 to have 5 f's but it only has 3.
It is adding the total amount of f's and attributing it to the first string.

Additionally, the expected strings are always prefixed with 1 or =. It never calculates the maximum letter count to be in the second string.
Examples:

Expected: "1:eeeeee/1:ffff/1:jjjj/1:llll/1:ddd/1:ttt/1:xxx/1:cc/1:pp/1:ss/1:zz/=:kk/=:ww",

Expected: "1:vvvvvv/1:eeee/1:bbb/1:ggg/1:ppp/1:zzz/1:dd/1:ff/1:ll/1:rr/1:tt/1:ww/1:xx/=:aa/=:cc/=:mm/=:qq"

Expected: "1:ddddddd/1:hhhh/=:aaaa/1:lll/1:ooo/1:qqq/1:xxx/1:bb/1:cc/1:gg/1:ww/=:rr/=:ss"

The example tests are consistent with the instructions and I was able to pass them.