This is similar to the bug #15022 that I submitted which has been fixed. However, this scenario is more complex. The table has <colgroup>, and its first row and first cell is using colspan. See provided attachment.
Created attachment 16157 [details] Testcase
I am unable to reproduce this bug in Safari 15.6 on macOS 12.5 using attached test case, when I click on "Change cell width to...", it shows following dialog "Cell 1's width should be 50px by now.. but it isn't :(" but once I dismiss dialog, the cell width is changed for "Cell 1" and it is similar across all other browsers (Chrome Canary 106 and Firefox Nightly 105). I am going to mark this as "RESOLVED WONTFIX", please mark this accordingly, if I am wrong. Thanks!