Using variable set in app designer callback for SCPI command in MATLAB function

Using app designer to run a function which includes a SCPI command:

function [measuredGain, measuredPhase] = VNA_ReadData(visaObj)
%
% Inputs:   visaObj - Object handler for VNA. Set using another function.
% Outputs:  measuredGain - gain measured from VNA in dB
%           measuredPhase - phase measured from VNA in degrees

% Retrieve Marker data for R&S VNA ZNB20 
M1 = query(visaObj, 'CALCulate1:MARKer1:Y?'); %ChannelNo=1, MarkerNo=1

% % Split gain and phase values into two value array
M1_new = split(M1,","); 

% % Convert strings into number and assign to output variable
measuredGain = str2double(M1_new(1));
measuredPhase = str2double(M1_new(2));
end

When run in the command window,

[measuredGain, measuredPhase] = VNA_ReadData(visaObj) 

successfully outputs measuredGain and measuredPhase.

However, when run in a callback in app designer,

[measuredGain, measuredPhase] = VNA_ReadData(app.visaObj)

where visaObj is a property, returns the error:

Undefined function 'query' for input arguments of type 'double'.
M1 = query(visaObj, 'CALCulate1:MARKer1:Y?'); %ChannelNo=1, MarkerNo=1

This appears to be due to a difference in visaObj in matlab and app.visaObj in app designer: The same 1x1 Visa, called visaObj, has different "ValuesSent" and "ValuesReceived" depending on whether it is called in MATLAB or App Designer.

   VISA-TCPIP Object Using RS Adaptor : VISA-TCPIP-0-169.254.168.142-inst0 %Visa Address of Network Analyzer in use.

   Communication Address 
      RemoteHost:         169.254.168.142 %IP Address - Network Analyzer is connected by Ethernet

   Communication State 
      Status:             open
      RecordStatus:       off

   Read/Write State  
      TransferStatus:     idle
      BytesAvailable:     0
      ValuesReceived:     10 % Discrepency in this value between app designer and MATLAB 
      ValuesSent:         305 % Discrepency in this value between app designer and MATLAB 

but the visaObj is set to base workspace in app designer using

assignin('base','visaObj',app.visaObj)

so they should be identical. Any suggestions as to why this is happening would be much appreciated.

Could it be the way I am sharing variables between app designer and a matlab function/script? I've also tried save/load but get the same error.