When editing a template parameter using VE, the editor adds text between the "=" and the space.
For example, if we have
Vertical Horizontal {{Template {{Template |param1 = |param1 = |param2 = |param3 = |param2 = }} |param3 = }}
VE shoud write this:
Vertical Horizontal {{Template {{Template |param1 = text |param1 = text|param2 = text|param3 = text |param2 = text }} |param3 = text }}
But, in fact, the actual edit is this:
Vertical Horizontal {{Template {{Template |param1 =text |param1 =text |param2 =text |param3 =text |param2 =text }} |param3 =text }}