Check if the content of HtmltextBox
is a valid markup and does not include unsupported tags and attributes - Supported HTML tags
and Supported CSS attributes
Note, that there might be differences in HTML rendering between design-time and when running the report. At design-time, the Design View of the HtmlTextBox item's 'Edit Expression' dialog is WYSIWYG editor and parses the HTML content you enter. At run-time the Reporting Engine has to handle the provided HTML input - that is not directly embedded in the generating document, instead we have a rendering that parses the values of each item.
In case, the issue still exists please attach a sample report demonstrating your current settings in a support ticket so we can give you further suggestions.
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and vote to affect the priority of the items