Tab characters in a text document can be interpreted differently by the program rendering the document, with different "tab stop" settings rendering them by different numbers of actual "blank space" characters (4 fixed spaces per tab, 8 fixed spaces, even possibly variable spacing, or even no spaces at all, as you are seeing, also depending on the fixed or variable font currently used in the rendering display program etc.).

So if you are committed to plain text (yay for you!) I would perhaps insert a specific number of spaces, not a tab character.