I2C - connecting 5V device to 3.3V host
I think I got it.

Many I2C devices connected to 5V and 3.3V I2C line (pullups connected to 3.3V) will not work.

High state for many devices is 0.7Vdd.



ADT7420 temperature will not work too - it has minimum high state level = 0.7Vdd.

For 5V Vdd minimum high level is 3.5V (above 3.3V). With 3.3V pullups 5V powered device will read line always as low state.

DS1307 is working, because minimum high state input level is 2.2V (below 3.3V).