"Variables should be used" unexpectedly shown when string variables used in middle of expression

 Hello,

  Here's in TS2016, when used a string variables in the middle of a expression, the analysis results of this sequece will report it as a unused variables like below in attaced picture, this leads me several times deleted the inuse varialbes and take trubble to recover them....Please help to check if any method to filter this kind of situation out?

  Thanks a lot~!