I've just setup the maxtrix authentication service and migrated my Homeserver to next-gen auth. After that, I turned on msc4190 in the bridges config file and the bridge was able to login successfully and send messages to me. However, messages I send do not get bridge, because the bridge hasn't received the decryption keys. In the log I can find the following, which doesn't really make sense to me, as there definitely is a device id in the request from the bridge:

synapse-signal                 | 2025-04-24T06:10:25.627Z DBG Sync response said we have less than 50 signed curve25519 keys left, sharing new ones... component=crypto keys_left=0 trace_id=06:10:25.627649
synapse-signal                 | 2025-04-24T06:10:25.647Z DBG Uploading one-time keys component=crypto count=100 trace_id=06:10:25.627649
synapse                        | 2025-04-24 06:10:25,648 - synapse.http.server - 130 - INFO - POST-229 - <XForwardedForRequest at 0x7cc0ee93c1a0 method='POST' uri='/_matrix/client/v3/keys/upload?device_id=ZOCFLMNPFE&org.matrix.msc3202.device_id=ZOCFLMNPFE&user_id=%40signalbot%3Ajannis-goeing.de' clientproto='HTTP/1.1' site='8008'> SynapseError: 400 - To upload keys, you must pass device_id when authenticating
synapse-signal                 | 2025-04-24T06:10:25.65Z ERR Failed to share keys error="M_UNKNOWN (HTTP 400): To upload keys, you must pass device_id when authenticating" component=crypto trace_id=06:10:25.627649