Your main issue is likely exposure estimation, not flower reflections. Sunny 16 is for direct sun; in overcast light or shade it will be off unless you adjust for the dimmer conditions. With negative film, scans can hide exposure errors because labs compensate, so check the negatives themselves: thin negatives usually indicate underexposure.

For flowers in overcast light, a handheld incident meter is the most reliable fix. Incident meters measure the light falling on the subject, so bright petals or dark backgrounds won’t mislead the reading the way reflective metering can.

If you keep using estimation, your samples suggest trying a small exposure adjustment from your current method and comparing results, but meterless guessing in cloudy light is difficult.

Also keep in mind that an older camera can have shutter-speed or aperture issues. If exposure remains inconsistent even after using a better metering method, have the camera checked—some cameras are accurate at some speeds but not others.

In short: use an incident meter if possible, don’t rely on Sunny 16 for overcast flower shots, inspect the negatives rather than scans alone, and consider a camera service if results still vary.