GPT-5 worse for custom GPT’s?

Have quite a few custom GPT’s and projects with instruction sets / loops and GPT-5 usually ignores the instructions / loops etc in my instructions.

Eg do step a,b,c with x and it ignores a,b,c and tries to solve x with a worse outcome.

This means generally a worse experience using GPT-5 in a custom gpt use case. Anyone else experience this? Any workarounds?