This is a Leaflet Bug.
This only happens when the "weight" of the polygon is a string: {weight: "10"}
The problem in leaflet is that it do some calculations with the weight and then while adding the weight to a value it is not calculated correct: 3 + "10" = "310"
and not 13
I created a PR for this issue: https://github.com/Leaflet/Leaflet/pull/7432
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