Thanks for posting this.
We have tried to solve this problem for differential equations with the function stack_strip_percent(ex,var) which replaces all variable names starting with % with those in var. See
See https://stack2.maths.ed.ac.uk/demo2018/question/type/stack/doc/doc.php/CAS/Differential_equations.md
Can you confirm this also works? If not, please email me a minimal and specific report and I will look into this further.
Chris