Can you please show up the three fall-time measurements?

Fall-time/rise-time = 2.2 * timeConstant, approximately.

It is necessary to understand to what extent the measurements are dissimilar, so a cause of imperfection can be sought for.

Internal resistance is usually very low compared to 10k, which dominates. The problem is likely to be due to capacitor nonlinearities. What sort of capacitor have you experimented with?

Troubleshooting first step boils down to evaluate how different the numbers are.

Fall-time/rise-time = 2.2 * timeConstant, approximately.

It is necessary to understand to what extent the measurements are dissimilar, so a cause of imperfection can be sought for.

Internal resistance is usually very low compared to 10k, which dominates. The problem is likely to be due to capacitor nonlinearities. What sort of capacitor have you experimented with?

Troubleshooting first step boils down to evaluate how different the numbers are.