The best fix is to prevent flicker during capture rather than rely on heavy post-processing.

For shooting stop motion, use:

• full manual exposure
• fixed aperture, shutter speed, and ISO
• consistent, non-changing lighting

If you let the camera auto-adjust, frame-to-frame differences can appear not only in brightness, but also in noise and other image characteristics.

For post-processing, sequence-normalizing tools do exist. One community suggestion was Granite Bay Deflicker, which is intended to normalize brightness across an image sequence and is available as a standalone tool or Adobe plugin. However, no firsthand experience with it was provided in the answers.

So the practical recommendation is:

1. Reshoot with manual settings and stable lighting if possible.
2. If you must fix an existing sequence, look for a deflicker/sequence normalization tool such as Granite Bay Deflicker.