There are usually 2 possible reasons for this:
A) Websheet found a cell with #ERROR or #NAME. When this happens, it usually tries to replace the given cell with the last known Block to work within that specific cell. To resolve this, make sure none of your cells are returning errors.
B) You may have a Block Formula hidden somewhere, e.g. in a group or inside a cell with white text. Double check to make sure this is not the case.