Artifacts in Temperature and Relative Humidity readings when powered via USB/5V

There is a problem when my device is powered via an external 5 V USB source: a lot of pulse-like artifacts appear in the temperature measurements and therefore also in the relative humidity readings, making these traces unusable.

An example is shown in the attached screenshot

The issue appeared after approx. 2 hours from the connection of the external supply. In the same time also the supply voltage trace changed: initially it settled to about 4.9 V, with some negative dips down to approx. 4.7 V (asociated to short charging periods that I cannot explain, because I expect here to see a constant voltage coming from the USB port).

Then, after 1 h and 50 minutes the supply voltage trace reverses to a floor of about 4.7 V with a series of short positive pulses going up to about 4.9 V. Here long charging periods are indicated. Starting from this moment the Temperature (and thus relative Humidity) traces show a lot of pulse-like artifacts.

What is happening? is there something I can do?

Thank you very much and best regards.

Alberto