You are referring to a right-aligned tab, not right indent. Very different animals.
The key here is the tab must be set to a point far enough to the right to accomodate the longest string you want to right-align.
but wierdly my right-aligned tab has plenty of room for the string which lines off it ...
it's the string that follows the tab (indented) which behaves randomly.
if it's just a bug in Indesign's tabs, can anyone suggest a workaround?
Are you sure the tabs are set the same for all paragraphs?
Looks to me like it might have more to do with justification settings than tabs.