My custom column width keeps changing

If you have three or more columns and set a custom width in the Column Settings editing screen, the other widths will readjust automatically. If you try to set a custom width on one of the other columns, the custom width you just set readjusts, and you can go around like that for a long time.

Solution:

There are drag handles on the edges of the columns that you can use to resize widths. Use those resize each column width to exactly what you want.