I've been using GPT 5.4 high (extra high on a few occasions) for planning and reviewing code. (I use GPT 5.4-mini for implementing the plans from 5.4). It's been great. Last week, I tried to resolve an issue with a home screen widget not displaying correctly on IOS. I tried twice with GPT 5.4 high. It couldn't fix the issue. I decided to give GPT 5.5 a try for the first time. It resolve the issue in one shot, it was pretty incredible.
However, in the past couple of days, I've noticed GPT 5.4 makes silly mistakes for example, it doesn't include tests for critical functions, for unit tests it doesn't mock correctly, some of the changes it proposes leads to build failures, etc. It didn't make mistakes like this before. This has caused me to start using 5.5 more often than I would like because of how expensive it is.
Am I the only one experiencing this?