We're seeing some random JSON tests failing. So far I've seen following types of issues:

• random Assert.Contains failing where string seem to actually contain value but yet assert fails - see my comment here for more details - most likely there are other CurrentCulture related APIs which could be affected
• MemoryStream ends up being corrupted, in the specific cases we've seen this it was related to async code paths, see last stack trace here - this is most likely some kind of memory corruption but it's pretty rare and hard to pinpoint
• random tests failing with incorrect serialization/deserialization issues - this is most likely due to JsonTypeInfos or other fields in the options being initialized in multiple threads. We've already fixed some of these but still some issues exist.

related:

• System.Text.Json failing some large file tests #59678
• Test failed at double initialization assert in System.Text.Json.Serialization.Metadata.JsonTypeInfo.InitializePropCache() #60962
• Assertion failed in System.Text.Json.JsonSerializer.LookupProperty #67816
• Assertion failure in System.Text.Json.Serialization.Converters.ObjectWithParameterizedConstructorConverter1.TryLookupConstructorParameter` #68584