Can you tell us how you have the 'switchtooldassembler' component?
The DSS entry SwitchToOldAssembler can be set to true to use 6.3 assembler.
Owning Ruleset: Pega-IntegrationEngine
Setting Purpose: SwitchToOldAssembler
The Old assembler is more stable than new assembler and we do not need to perform regression testing after making this change. So, switching to old assembler should work without any issues. The new parse assembler has many issues and it does not support parsing of more than 2 levels.
There is plan to replace/re-code the new assembler completely in future. The discussions are currently on going in this regard. Moreover, there is also BUG logged already in our system related to the reported issue.
Ideally the behavior of Old assembler should be same as new assembler. It is recommended to use the below Dynamic System Setting entry to resolve the issue: