OBS Studio multitrack video via RTMP cannot authenticate?

[Kenkoman]

Question

hey, I can't tell if this is an issue regarding OBS Studio or MediaMTX but when I try to stream RTMP using OBS Studio while defining a custom JSON override for multiple video tracks, authentication appears to always fail on MediaMTX if my mediamtx server requires authentication to publish a stream.

im trying this feature of OBS Studio using this guidance written here: https://mediamtx.org/docs/publish/obs-studio#multitrack-video - when streaming RTMP using OBS Studio normally, authentication defined in OBS's "Custom..." service stream settings like rtmp://hostname/streampath?user=name&pass=password works successfully with "Enable Multitrack Video" disabled. but as soon as this is enabled, mediamtx always rejects the stream like so:
[RTMP] [conn 54.133.177.101:58237] closed: authentication failed: authentication failed
this is the same error usually returned when the provided authentication details are incorrect or missing, but the stream URL has not changed whatsoever in this config.
when my mediamtx server configuration is changed to no longer require authentication to stream, OBS is able to stream perfectly fine. but i need to keep my server passworded.

i've looked in OBS's logs and it does appear to still be connecting to the same URL including authentication details, so i can't tell what's going wrong here. this feature of OBS would be really great for defining custom multitrack setups (for example; streaming both high quality AV1 and more compatible H264 simultaenously is something i need) so any help is greatly appreciated!