Welcome to the forum!
First off I do not use the API.
This may not be a bug by the way some of GPT-5 models work. Specifically the JSON output might be a result of changes for CFG (related to structured outputs).
While I can not directly answer your questions, I am thinking that with the changes to GPT-5 models, e.g. CFG, the models now create a more specific form of JSON via CFG and thus the nice formatting is no longer returned.
I also suspect that if you provide a different CFG for creating JSON, it should be able to produce the nicely formatted JSON you seek.
At present this is the best related post to CFG on this forum
In particular pay attention to the replies by Michal_Moskal
HTH