Could you please complete the following steps in order to detect the problem and find its solution:
1) Activate a default WordPress theme for a while (Twenty Nineteen or Twenty-Twenty) and check if the problem remains.
2) Deactivate all of your plugins except the Sitemap plugin, and also check if the problem is still present. If it is not, activate the plugins one by one, meanwhile checking, after which plugin activation the problem appears again.
3) Contact us back with information about the results.