It happens most of the time at least for me (I use the free version) because it is hard for the LLM I suppose to comprehend the word count. Did you try selecting reasoning and then trying, It might resolve the issue but not sure.
ChatGPT cannot count words or produce word-count-limited text - #16 by joseicarobc is a great community post you can check out that discussed this same issue and how it can be resolved.
Hope this helps 