H̶i̶,̶ ̶ ̶
A̶f̶t̶e̶r̶ ̶u̶s̶i̶n̶g̶ ̶t̶h̶i̶s̶ ̶a̶p̶p̶ ̶I̶ ̶r̶a̶n̶ ̶i̶n̶t̶o̶ ̶t̶h̶e̶ ̶f̶o̶l̶l̶o̶w̶i̶n̶g̶ ̶i̶s̶s̶u̶e̶.̶ ̶3̶D̶S̶ ̶M̶H̶X̶X̶ ̶-̶>̶ ̶M̶H̶G̶U̶ ̶S̶w̶i̶t̶c̶h̶ ̶w̶o̶r̶k̶s̶ ̶p̶e̶r̶f̶e̶c̶t̶.̶ ̶I̶ ̶l̶o̶a̶d̶e̶d̶ ̶t̶h̶e̶ ̶f̶i̶l̶e̶ ̶i̶n̶-̶g̶a̶m̶e̶ ̶w̶a̶s̶ ̶a̶b̶l̶e̶ ̶t̶o̶ ̶s̶a̶v̶e̶ ̶w̶i̶t̶h̶o̶u̶t̶ ̶a̶n̶y̶ ̶i̶s̶s̶u̶e̶s̶ ̶o̶n̶l̶y̶ ̶c̶r̶a̶s̶h̶e̶s̶ ̶w̶h̶e̶n̶ ̶y̶o̶u̶ ̶o̶p̶e̶n̶ ̶D̶L̶C̶ ̶e̶v̶e̶n̶t̶s̶.̶ ̶ ̶
M̶H̶G̶U̶ ̶S̶w̶i̶t̶c̶h̶ ̶-̶>̶ ̶3̶D̶S̶ ̶T̶h̶i̶s̶ ̶i̶s̶ ̶w̶h̶e̶r̶e̶ ̶t̶h̶e̶ ̶p̶r̶o̶b̶l̶e̶m̶ ̶b̶e̶g̶i̶n̶s̶.̶ ̶F̶i̶r̶s̶t̶ ̶o̶f̶f̶ ̶t̶h̶e̶ ̶f̶i̶l̶e̶ ̶s̶i̶z̶e̶ ̶i̶s̶ ̶r̶a̶t̶h̶e̶r̶ ̶s̶t̶r̶a̶n̶g̶e̶ ̶i̶t̶ ̶s̶t̶a̶y̶s̶ ̶t̶h̶e̶ ̶s̶a̶m̶e̶ ̶f̶i̶l̶e̶ ̶s̶i̶z̶e̶ ̶a̶n̶d̶ ̶t̶h̶e̶ ̶s̶w̶i̶t̶c̶h̶ ̶s̶a̶v̶e̶ ̶f̶i̶l̶e̶.̶ ̶I̶ ̶t̶h̶i̶n̶k̶ ̶t̶h̶i̶s̶ ̶m̶i̶g̶h̶t̶ ̶b̶e̶ ̶p̶a̶r̶t̶ ̶o̶f̶ ̶t̶h̶e̶ ̶p̶r̶o̶b̶l̶e̶m̶.̶ ̶F̶u̶r̶t̶h̶e̶r̶m̶o̶r̶e̶ ̶o̶n̶c̶e̶ ̶t̶h̶e̶ ̶S̶w̶i̶t̶c̶h̶ ̶s̶a̶v̶e̶ ̶h̶a̶s̶ ̶b̶e̶e̶n̶ ̶c̶o̶n̶v̶e̶r̶t̶e̶d̶ ̶t̶o̶ ̶3̶D̶S̶ ̶i̶t̶'̶s̶ ̶n̶o̶ ̶l̶o̶n̶g̶e̶r̶ ̶o̶p̶e̶n̶a̶b̶l̶e̶ ̶o̶r̶ ̶r̶e̶c̶o̶n̶i̶z̶e̶a̶b̶l̶e̶ ̶b̶y̶ ̶t̶h̶e̶ ̶s̶a̶v̶e̶ ̶e̶d̶i̶t̶o̶r̶.̶ ̶C̶i̶t̶r̶a̶ ̶i̶s̶ ̶a̶b̶l̶e̶ ̶t̶o̶ ̶l̶o̶a̶d̶ ̶t̶h̶e̶ ̶f̶i̶l̶e̶ ̶b̶u̶t̶ ̶n̶o̶t̶ ̶a̶b̶l̶e̶ ̶t̶o̶ ̶s̶a̶v̶e̶ ̶b̶u̶t̶ ̶f̶o̶r̶ ̶s̶o̶m̶e̶ ̶r̶e̶a̶s̶o̶n̶ ̶i̶t̶ ̶s̶a̶v̶e̶s̶ ̶f̶i̶n̶e̶ ̶o̶n̶ ̶a̶ ̶r̶e̶a̶l̶ ̶3̶d̶s̶ ̶t̶h̶o̶u̶g̶h̶.̶ ̶ ̶A̶n̶y̶ ̶c̶h̶a̶n̶c̶e̶ ̶
f̶o̶r̶ ̶a̶ ̶f̶i̶x̶?̶ ̶:̶)̶
I have actually found a fix. It takes a little bit of logic. So, when you open the MHGU (Switch) save file and convert it to 3DS and save it becomes unreconizeable by the save editor next time you open it. The error lies within the file size, the actual data conversion actually went ok. So, the MHGU file size is 5.039kb, but the 3DS save file is 4.616KB when you convert your switch save file to 3DS the file size becomes 5.039kb larger than usual which causes all different kind of problems like save editor not being able to read it or Citra not being able to save thinking the file is corrupted, Citra can still read the save however.
The fix to the problem is that at the end of the file there's too many excessive lines which the 3DS file doesn't need. It might just be a reservation for the Switch. How I was able to fix my file is removing the excessive 0000 at the end of the file. You need exactly 32 full lines of 0000. I can give you an example below. This is the best work around till OP updates the app.