Interesting to hear about a similar problem. Although just to clarify, in my case the sysex isn't coming from the TX81Z. The sysex is getting to the Minibrute because I have it on the same MIDI connection as the TX81Z, and I sometimes want to send patch data to the TX81Z. I'm thinking of adding another device or two on the same connection down the road, so I hope the Minibrute will ignore any sysex that I send on the connection that it doesn't recognize.
One might suggest putting the Minibrute on its own connection, but I only have so many midi outputs so I like to share each output among multiple devices that only listen to one or a few channels. Originally I had the MB connected to the other unit's MIDI Thru. I thought there was a chance that might be problematic, so as a test I tried connecting directly to the Minibrute, but it still "locked up" when I sent the sysex to that connection.
I'm hoping someone from Arturia will investigate. It really seems to me like a bug that the Minibrute isn't ignoring the message.