Hello, Thank you for reporting the bug. Unfortunately without clear steps to reproduce it, we cannot track down the origin of the problem. Please provide a clearer set of step-by-step instructions on how to reproduce the problem.
Created a spreadsheet in Calc. Merged the first 6 cells then entered a text string in the merged cells. With the merged cells selected, clicked on Format, Cells and then the Alignment tab and tried to vertically align the text string. The text string becomes unreadable after clicking OK until double-clicking back on the merged cells with the text string. Result seems to be that you cannot vertically align column headers in this version of LibreOffice.
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Most likely the same as Bug 94917. *** This bug has been marked as a duplicate of bug 94917 ***