@Orvillain I think the problem is that HISE converts Parameter IDs into actual attributes. This is only the case with hardcoded modules, script processors or DSP networks properly escape that in the value string.

<Processor Type="Hardcoded Master FX" ID="HardcodedMasterFX1" Bypassed="0"
           Network="No network" YourParameterGoesHere="0.5" TryValidating(That)="nope">
  <EditorStates BodyShown="1" Visible="1" Solo="0"/>
  <ChildProcessors/>
  <RoutingMatrix NumSourceChannels="2" Channel0="0" Send0="-1" Channel1="1" Send1="-1"/>
</Processor>

I do a bit of sanitizing at some place though (eg. remove white space for the XML attribute, so all my ramblings might be moot because I sprinkled a character sanitation in there too.

ahhhhhhhhhhhhh, gotcha. Yes, then that does make sense that parenthesis would possibly break things... and now I'm going to do a sweep through my code to see how many landmines I've invented.

<Processor Type="Hardcoded Master FX" ID="HardcodedMasterFX1" Bypassed="0"
           Network="No network" YourParameterGoesHere="0.5" TryValidating(That)="nope">
  <EditorStates BodyShown="1" Visible="1" Solo="0"/>
  <ChildProcessors/>
  <RoutingMatrix NumSourceChannels="2" Channel0="0" Send0="-1" Channel1="1" Send1="-1"/>
</Processor>

I do a bit of sanitizing at some place though (eg. remove white space for the XML attribute, so all my ramblings might be moot because I sprinkled a character sanitation in there too.

@Orvillain Strange, I am inclined to agree with you since I've done this with other nodes, but removing them in this particular context seems to have solved the problem. Still I can't find the root issue here so I'll keep digging...

That is very odd. There's nothing in the XML specification that says you shouldn't use parenthesis in an attribute name, so far as I know. The only character to definitely be wary of in my experience is an ampersand.

        void createParameters(ParameterDataList& data)
        {
            {
                parameter::data p("PreGain (dB)", { -24.0, 24.0, 0.1 });
                registerCallback<0>(p);
                p.setDefaultValue(0.0);
                data.add(std::move(p));
            }
            {
                parameter::data p("Hold (ms)", { 0.0, 100.0, 1.0 });
                registerCallback<1>(p);
                p.setDefaultValue(5.0);
                p.setSkewForCentre(30.0f);
                data.add(std::move(p));
            }
            {
                parameter::data p("Release (ms)", { 1.0, 2500.0, 1.0 });
                registerCallback<2>(p);
                p.setDefaultValue(80.0);
                p.setSkewForCentre(800.0f);
                data.add(std::move(p));
            }
            {
                parameter::data p("Ceiling (dB)", { -60.0, 0.0, 0.1 });
                registerCallback<3>(p);
                p.setDefaultValue(0.0);
                data.add(std::move(p));
            }
        }