Why have the lines on the stock HR widget gone chunky?

The linework for the axes and datapoints on the stock HR chart widget and the Temperature widget have started to be displayed with a really thick pen. If I go into the HR 7 day history chart and back out again it resets to normal, but reverts to the chunky lines if I select any other widget or return to the watch face.

Fenix 5, Software version 25.00