[Mario Sosa]

Was having the same issue. It was occurring on Pro Res HQ footage that was being export with h264 codec. There were two solutions. One was to set video processing to always render in RGB, and then export. The other, was on quicktime conversion, after selecting the h264 codec and adjusting settings, select filter, and add the brightness and contrast filter. Leave the settings at zero. For some reason this keeps the brightness level consistent across clips that have rendered sections (like text, transitions).