I have created various testing scripts using litellm + bedrock and the same model name that you are using, and I cannot replicate the issue; I see cost details for each of my spans.
I will have a short period of availability from 2:00pm est that we can try for a call.
In the meantime, if you can run this test script and verify that the spans produced also do not display cost in Phoenix that would be very helpful. You may have to tweak the Phoenix connection arguments in the instrumentation if you are not running a local phoenix instance or have authentication enabled. The script uses uv, but you may be able to execute with python directly if you have the required dependencies already installed in a virtual env.
uv run litellm_bedrock_anthropic_async.py