The problem is that ts_salobj's ReadTopic adds an extra filter for "ackcmd" data, in order to weed out acknowledgements to commands issued by some other Remote. That's important for normal operation but not wanted for this situation.
My fix was to add a new filter_ackcmd argument to salobj.ReadTopic. This defaults to True, for the old behavior, but ts_salkafka overrides it. I added a unit test whose doc string includes this ticket number.
Pull request: https://github.com/lsst-ts/ts_salkafka/pull/14
Note that this change requires ts_salobj 6 – which requires ts_sal 4.2. So using the fix awaits release of ts_sal 4.2. If that gets badly delayed I can release a new version of ts_salobj 5 that cherry-picks the necessary change.