This issue was originally reported for js-bson but is also present for bson-ext. The js-bson fix should apply to bson-ext, but we should double check and add the appropriate tests.
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https://github.com/mongodb/js-bson/issues/334

Hello. Decimal128.fromString is working incorrect, you can reproduce bug using negative numbers of pattern 0.00XX :

bson.Decimal128.fromString('-0.0097').toString() // -0.0000
bson.Decimal128.fromString('-0.0011').toString() // -0.0000

Same numbers, but with different zero count will work:

bson.Decimal128.fromString('-0.00970').toString() // -0.00970
bson.Decimal128.fromString('-0.00110').toString() // -0.00110
bson.Decimal128.fromString('-0.00010').toString() // -0.00110

bson package version is 4.0.2