It is because the "forwarded index" attributes are only applicable under the global [tcpout] stanza. This filter does not work if it is created any where else
Your configs should be
[tcpout]
indexAndForward = true
forwardedindex.filter.disable = false
forwardedindex.0.blacklist = index_1
forwardedindex.1.blacklist = index_3
forwardedindex.2.whitelist = index_2
[tcpout:indexerB_9997]
disabled = false
server = indexerB:9997
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