If you search trap ringing in this forum, you can find multiple posts discussed about this and with answer from Mike Engelhardt (author of Qspice).
Your example demonstrated how trap ringing can be observed. Spice uses numerical method and trapezoidal/gear integration method and they exhibit unique characteristics. In default, Qspice uses trapezoidal integration and trapezoidal ringing is one of its characteristics.
As your simulation is basically function and formula, not a circuit with realistic components, where function being integrated has sudden changes, and this is why trap ringing is observed. These slides explains why trap ringing in your setup (I simplified your circuit with a source disabled its timectrl). Change to Gear can eliminate this ringing, but remember this is not a recommended method as it changed your circuit by introduces artificial damping.
