Can't input MIDI in GM-emulator VSTs properly

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Originally posted by musikbear, see end of issue. Rewriting. Originally reported by me and another user on the LMMS forums here.

About the VSTs

There exists VSTs which act like GM emulators: You can give MIDI input into it, and if it works properly, it can act like a GM soundfont.
Now, the lmms forum thread linked to lists at least two VSTs of this sort which don't run properly, or as expected.
(I only have the SYXG, and so I cannot speak for the Edirol Roland Orchestra VST.)

• Yamaha- S-YXG50 VST
• Edirol Roland orchestra 1.03 VST

Expected Behaviour

(As far as the S-YXG 50 is concerned:)
There should be some way to input MIDI to the VST such that it can product all sorts of sounds.
VSTHost by HermannSeib seems to run this VST as expected, to maximum capability. See:
https://drive.google.com/file/d/16MFhPKTk8pOniv1N98G-vDV_2mWU8UAE/view?usp=sharing
("noprogram.mid" from here is the MIDI file used.)

Actual Behaviour

(As far as the S-YXG 50 is concerned:)
I can't seem to use any patch other than 0: Grand piano and 128:0: Standard drumkit.
I can use the latter: in the MIDI tab of VST instrument, if I tweak the "MIDI Output" dial, I can change which channel the sound shows up in on the VST GUI.

But note that all other MIDI output channels seem to send only to grand piano.

Now, I don't believe there's supposed to be a specific way to "access" these banks, but I believe there should be a way to use them through controlling what sort of MIDI goes into the VST.
There are no modifiable parameters in the parameter control array in LMMS GUI, but rest assured there are none in VST Host either, so there probably aren't supposed to be any.

Originally posted by @musikBear in #3918 (comment)

My sincerest apologies about the unusual formatting: I created it as a "Reference to this in new issue," and it didn't assign me a label and template. Once created, I referred to a bug report template to rewrite this, and it didn't work quite well; but using it as reference I have tried my best to organize this issue as I see fit.