https://pypi.org/project/msgpack/1.0.0/ -- see the section, "Major breaking changes in 1.0":

use_bin_type=True by default. bytes are encoded in bin type in msgpack. If you are still sing Python 2, you must use unicode for all string types. You can use use_bin_type=False to encode into old msgpack format.

if use_bin_type=False isn't passed to FluentSender in msgpack_kwargs, fluentbit will see that the first element of each received message is not of type MSGPACK_OBJECT_STR, and refuse to enqueue the message. If you have debug output turned on in fluent-bit, you should see it output a message like,

[2020/02/20 21:19:55] [debug] [in_fw] parser: invalid tag format, skip.

I have verified that this happens in fluent-bit 1.3.5, and while I haven't tried it using 1.3.8, the relevant code in that release appears unchanged.