Perl Excel Writer XLSX merge_range_type won't render properly in Excel

Question

I'm writing a calendar app that merges a span of cells across a row to be formatted with a colored background and rich text string. Everything renders beautifully in LibreOffice Calc but Excel gives a brief error msg (Removed Records: Merge cells from /xl/worksheets/sheet1.xml part) and does not render a row merged into one cell. Instead it renders into groups of four cells.

The two images describe my problem.

When creating this calendar I wrote out the calendar outline or frame then the numbering. Then I tried to overlay the event data week by week on top of the outlne thinking it would work. I think Excel doesn't like my additive approach to creating this calendar but I'm not sure what I need to do.

This is the line that should be merging everything: $worksheet->merge_range_type( 'rich_string', $y1,$x1, $y2,$x2, @txt, $chosen_fmt );

Thinking I should be zeroing out the cells beforehand, I tried:

    my $fmt_clear = $self->SUPER::add_format(
        font    => 'Arial',
        size    => 10,
        color   => 'black',
        bottom  => 0,
        top     => 0,
        left    => 0,
        right   => 0
    );

    for( my $x=$x1; $x<$x2; $x++ ){
        print "$y1,$x ";  # Double checking coordinates
        $worksheet->write( $y1, $x, ' ', $fmt_clear );
    }

This didn't work. No matter what I tried I got the LibreOffice to render properly and Excel messed it up. And I triple checked the row/column counts to eliminate an off-by-one error. And caching wasn't an issue, I printed a timestamp beneath each calendar (not visible in img). I would like the Excel version to render properly like the LibreOffice version. Thank you.