Buddy box only works one way? (Taranis and Horus)

[Chuppster]

I recently upgraded from a QX7 (Opentx 2.3.9) to a Horus X10S (Opentx 2.3.9). I want to set up a buddy box with the Taranis as slave and the Horus as master. I plugged a cable into each trainer port and set the Taranis to a default model with the "trainer" option set to "Slave/Jack". When I go into the "Trainer" page in the system menu of the Horus the input indicators at the bottom of the screen don't change. However, if I do the reverse (Horus in slave mode) I see the Taranis receiving input from the Horus. I tried to rule out the Taranis by plugging my old buddy box TX (FlySky I6X with a cable to convert to a 3.5mm) that works as a slave for my Taranis into the Horus, but it makes no difference.

Am I missing some crucial setting to get the Horus to work with a wired buddy box?