I set variable "a" as a secret value under configuration A, and I added a promotion association to configuration B under configuration A.
After A triggered B run, I was able to see the plaintext of variable "a" on the "Variables" page of B build results.
You need to tell configuration B that it is also secret. To do it, define a as a secret variable in B. Or you may move the definition of "a" in common parent of A and B, to avoid redefine them in A and B separately.