There are at least 2 major issues with JSON support in Qt 5.15.0:

1 - JSON objects that have a key that maps to a "null" value get omitted from the serialized JSON.  E.g.:

 

QVariantMap{"key1","somevalue,"key2", QVariant{ }}

 

Serializes to the following JSON on Qt 5.14 and below:

 

{{

{"key1":"somevalue","key2":null}

}}

 

On Qt 5.15.0 and above it produces:

 

{{

{"key1":"somevalue"}

}}

 

Notice how "key2" is omitted.  This is a breaking change because sometimes existing application specifically put null values into a json object to indicate something.  What's more, if you attempt to implement JSON RPC and you sent a null "result", the resulting produced json is no longer valid JSON-RPC! 

 
2 - QByteArray's that get put into QVariantMaps and QVariantLists end up auto-converted using base64 encode.  This is unexpected behavior and a breaking change from previous versions of Qt. This breaks many applications.

I consider these pretty bad regressions.  I urge you to fix them.  Especially (1) which has no workaround and makes generating some shapes of JSON absolutely impossible.