I just had 10 pcs new LYWSD03MMC B1.9 thermometers. All of those have very similar humidity and temperature values in same placement with stock firmware. I flashed most of those sensors one by one to with latest firmware version 3.4. Always after flashing humidity value was decreased instantly by 11-12%. This is only concerning humidity, I do not see any relevant change in temperature value after firmware update.

I have had now all 10 thermometers in Sodium Chloride calibration box about >12 hours and thermometers with stock firmware shows 71-72% that is very close. All thermometers with pvvx firmware shows about 60%. Correct value for calibration should be 75.4%. Meaning that stock firmware is only 3-4% too low compared to calibration result (after 12h, I will still wait for >24h for final result), but pvvx firmware shows about 15% too low humidity value. I am using this procedure for humidity calibration:
https://www.vernier.com/til/4337
https://www.youtube.com/watch?v=aeLPRZQTA4w

I could use humidity offset to calibrate sensors but firmware config does now allow to set bigger values than 12.7%. If bigger value is given it will be set as 12.7%.

Also temperature and humidity values are not rounded in LCD or in advertisement (using Atc1441 because less digits). If temperature shown in Telink flasher is 23.87 it shows 23.8 in LCD and advertisement. Value 23.87 should be rounded to 23.9.