This is a bit of a strange issue, possibly only happening on Linux. I have USB-C connected to computer, but nothing is connected/using the FH-2 MIDI input on the Linux side. Not in ALSA, JACK nor PipeWire. If I start either internal clock on FH-2, or MIDI clock is forwarded to USB-C from another connection, eventually FH-2 will report send queue overflow. And this is for the USB-C connection, even though it does not say that. Any data presumably works. It can be resolved with starting something in Linux that uses the FH-2 MIDI input, such as aseqdump, and "polls" the "USB queue" (I don't really know how that works). Or of course disconnecting the USB-C cable. If I start something, again aseqdump, after this has happened (without disconnecting from computer), it will actually receive the MIDI data that FH-2 has buffered while nothing was using the input on the Linux side. I don't know how USB works and what mechanisms are available to know whether someone is "listening" on MIDI data, but I figure this is a bug in FH-2.
https://www.modwiggler.com/forum/viewtopic.php?p=4211126#p4211126