I just tried it twice on an item and the two affixes stayed exactly the same…
Thank you for the report!
Could I ask for a screenshot of the item in question?
They are not guaranteed to change. A Reroll may result in any number, including the previous one.
Some affixes have very small range of numbers per tier (glancing blow, T5 ranges from 12 to 15 afair, that’s only 4 numbers). This makes odds of getting same result several times larger.
You only tried twice, how about 20 times?