This would indeed be serious if replicable, but I'm unable to replicate. In my test script, I added an If statement that tests for:
Let( $test = 12345; 1 )
I run the script.
I open data viewer and watch/evaluate for $test, but the value is null.
Please elaborate on what step your Let() statement is in and show us what that statement says.
A few versions back (circa 12 maybe?) I encountered a similar situation where I found a local variable still hanging around after a script ended. Ever since then, I've tried to be meticulous about resetting local variables when a script is about to use them. I suppose that practice might also help avoid problems if you ever end up sandwiching two scripts together and their variable names overlap, but I mainly did it out of concern that I couldn't trust local variables to be 100% clean going in.