With manual exposure and manual strobes, the camera should not be changing exposure on its own. The most likely causes are setup-related, not hidden auto-exposure.

Common causes mentioned:

• auto white balance can make images look brighter/darker or shift overall appearance
• ambient light may be contributing if it isn’t low enough relative to flash
• larger subjects can block light, cast shadows on the white background, or change how light reflects back into the scene
• lights or subject distance/position may be changing between products
• some flashes can sleep/reset if they are speedlites rather than consistent studio strobes

The odd EXIF lens data is likely just reporting zoom-position info and probably isn’t the cause of exposure changes.

To improve consistency:

• set white balance to a fixed value, not AWB
• keep ambient light low so flash is doing all the work
• keep lights, subject, and background distances consistent
• check that your flashes are firing at the same power every shot
• use a gray card/reference shot to standardize exposure and post-processing

If bigger objects need different placement or lighting, some variation is expected unless the lighting setup is adjusted to compensate.