Hi @Sian Milne,
I greatly appreciate your detailed information of the issue.
Based on your description, this is likely a recent rendering/UI issue within SharePoint and there are several feedback from other users in the community said they had experienced the same behavior. The issue is similarly reported as the SharePoint List Date picker was narrowed and only displayed 4 days per week, almost the same as the situation you're experiencing now. You can find more details in these posts where other users have shared the same experience on this issue:
In this case, I strongly recommend you submitting feedback directly to Microsoft via SharePoint feedback portal or you can upvote the below feedback from a user who has reported the similar issue. The more upvote it has, the more visibility it has which helps the product team aware of the issue and develop a remediation plan as soon as possible.
Link to the post: SharePoint List Date and Time Picker Only Shows First Four Days of the Week in Standard Add Item Fo…
As a forum moderator, I work independently of the product team, and I don't have visibility into internal design decisions or the ability to adjust how the feature behaves. I can only help confirm how the product currently works and make sure you have clear information.
I'm sure there are other users who have been in the same situation but don't know where to share their feedback, so hopefully you will click "YES" under this post if you can. This helps pin the post to top, making it easier for other users who may be experiencing the same issue to find this post, understand the context, and support the feedback. Your voice matters, and collective feedback is one of the most effective ways to drive meaningful improvement.
If you have any other questions or need any further assistance, please leave a comment here and I will be happy to continue to help you.
Thank you for your cooperation and understanding. Wishing you a wonderful day ahead.
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