Matrix Column Widths

Hi jthom91 
Thanks for sharing the screenshots and additional testing .

This appears to be related to a recent change in the Matrix visual rendering behavior, where column widths can be recalculated even when Auto-size width is turned off. There are similar community reports documenting the same behavior:

Matrix Table resizing columns with Autosize column... - Microsoft Fabric Community
Re: Matrix visual column width rendering regressio... - Microsoft Fabric Community

In the current behavior, removing the helper/spacer measure causes the Matrix visual to re-render and recompute column widths, so the earlier workaround (adding a REPT measure, sizing, then removing it) no longer persists after the measure is deleted.
As a workaround keep a spacer measure permanently in the visual, for example:

Column Spacer = REPT(UNICHAR(8203), 12)

Add this measure to the Values section, enable Show values on rows, turn Auto-size width = Off, then manually resize the columns once. After that, keep the measure in place and hide the spacer row by using matching font/background colors, turning off subtotals, and reducing row padding.

Unfortunately, there is still no native option to enforce equal column widths in Matrix visuals, and the newer “Grow to fit” behavior distributes space based on content size rather than evenly across columns.
 
There are existing Ideas related to fixed/equal column widths and retaining manual sizing. I recommend upvoting them, as this helps the product team prioritize improvements:
Enable setting a fix width of each column in a mat... - Microsoft Fabric Community
Fixed Column Width for Columns in PBI - Microsoft Fabric Community
Enable setting a fix width of each column in a mat... - Microsoft Fabric Community

If you have any further questions, feel free to reach out and we'll be glad to assist.
 

Regards,

Microsoft Fabric Community Support Team.