A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Dear Charlotte,
Thanks for writing back and updating with us. Apologies for the delay in the response.
As per the further updates, I believe the users should be having higher permission level as well assigned to their Office 365 account because when the user is assigned with the two different permission levels, the SharePoint Online will consider the higher-level permission.
So, I request you to check the permission of the user who is assigned with no delete permission but still able to delete option via below steps:
Open the SharePoint Online site> Settings> Site Permissions> Advanced Permission Settings> Check Permissions> Enter the user email address> Check Now> Take a complete screenshot and share it with us.
If the user is assigned with the Edit permission or Contribute permission, please go to the Permission Levels> Click on the Permission> Take a screenshot and share it with us.
Appreciate your patience with us and have a wonderful day!!
Best Regards,
Sophia