Which Ardour version? It works for me in 8.1…
Also which platform is this (mine is Linux) and how did you install the Surge XT plugin in the first place (I did using the platform package manager, apt in my case)?
Last, but not least - are you sure you want more than one instrument plugins (Ace Fluid Synth and Surge XT) on one track?
It works for me as well (macOS and Linux). @x42 the current surgeXT deliverable installs both, the instrument AND an effect.
But the thought makes sense. If these are remnants of an old bug, maybe removing all traces of surge XT, Cache deletion and rescan might solve this?
I installed Ardour through debian standard repository (apt install ardour), and
I installed Surge XT by downloading surge-xt-linux-x64-1.2.3.deb from https://surge-synthesizer.github.io/downloads (https://github.com/surge-synthesizer/releases-xt/releases/download/1.2.3/surge-xt-linux-x64-1.2.3.deb) ; and then :
apt install ./surge-xt-linux-x64-1.2.3.deb
It looks me strange having more than one instrument in a track, I thing that something is wrong.
With 8.1. Surge XT works fine. Just make sure you use the VST3 version, the LV2 version has issues storing its state and reverts to the init state when you reopen a session that uses the LV2 version (as far as I can see due to the JUCE version used, not LV2 itself).
Another idea what might be going on.
Try choosing Surge directly in the add midi track menu.
When you add surge on a track where fluid synth is already inserted, the pin connections might be wrong. I am not infront of a computer to verify rn, so just an idea. The Ardour drop-down menus are a little confusing as most of them have no search or type to jump.
Note that that is still old as Ardour is on v8 now. You can install from the website, and probably should, but at least that gets you more modern significantly and many bugfixes since that other version. Just keep in mind there will be that much more work between what you are running now and the current. So if you run across bugs, the first step is always going to be to check with a version from this website, even the free demo version and see if it persists there.
As I have said before, I am using Debian 12 with package for Ardour 7.3. I prefer the major release stability of Debian Linux to a rolling distribution of Linux because there is always something not working on a rolling release and I am not a Linux system administrator. I have to wonder if Ardour 8.x on Debian 12 might not work because of dependencies on new libraries not part of Debian 12 and possibly not compatible with Debian 12. I don’t know anything specific about Ardour’s dependencies, but I do know that I avoid problems by using Debian stable.
The versions of ardour that we provide via Download Ardour | Ardour Community have no system dependencies except a C++ standard library and Xlib. They do not care what distro or version of a distro you are using (more or less).