One solutions could be:

• Anticorruption Layer returns 504.
• Microservice B retries 3 times, and after that returns 500 instead of 504.
• Don't retry on 500 in Microservice A or BFF.
• Every service should have Retry policy and Circuit breaker.

Another solutions could be:

• Anticorruption Layer returns 504.
• Microservice B retries 3 times, and after that sends 504 back with some header (Retry-Count:3)
• In parent service don't retry if Retry-Count is more then 3.

Does it make sense?