About lap 1/2 pitstops.
Team wanted to stay out, they told Lando that the rain was easing off. This is why they stayed out, expecting that loss from two pitstops inter changers will be bigger than time lost from running on softs.
Next lap around Lando was pushing for tyre change, team told him to stay out if he can keep it on track, he asked for tyres, they accepted.
Piastri did not push for inters and thus he stayed out. After inters fell off, Piastri was P8 with Lando in P12 or so. But then Piastri flatspotted his tyres trying to overtake Magnussen, he pitted and then safety car happened.
In short, Lando wanted to pit but team wanted to stay out. They underestimated effect of the rain so they never thought about pitting on lap 1.