One of the Sitecore custom field stopped working after we applied Cumulative hotfix for Sitecore 9.3 on CM server. We started code review and debugging but didn’t find any relevant information which could be the reason of this unexpected behavior.

Finally we raised a support ticket with Sitecore and found that this issue seems to be due to security fixes in the cumulative hotfix. You can find more detail on this issue in this Sitecore KB article:

https://support.sitecore.com/kb?id=kb_article_view&sysparm_article=KB1003018#FAQ

As per recommendation from Sitecore support team, we applied below patch configuration on CM server:

<?xml version="1.0" ?>
<configuration xmlns:role="http://www.sitecore.net/xmlconfig/role/"
xmlns:security="http://www.sitecore.net/xmlconfig/security/"
xmlns:patch="http://www.sitecore.net/xmlconfig/">
    <sitecore>
        <reflection role:require="ContentManagement or Standalone">
            <allowedMethods>
                <descriptor type="ProjectName.Sites.SitecoreCode.Fields.CustomFieldName"
                methodName="Add" assemblyName="ProjectName.Sites.SitecoreCode" />
            </allowedMethods>
        </reflection>
    </sitecore>
</configuration>

The related XAML controls are not used on CDs since hotfix contains instructions to disable the related handler so we applied this patch only on CM server. Hope this helps!

Environment:
Sitecore 9.3 Initial Release

Happy Learning !!!