I'm so sorry about your first pup.
That post is really low.
I personally would retest in one month making sure that she doesn't get any meds (including heartworm, flea and vaccines) for two weeks before. These can skew the results. Idexx says the post shouldn't be over 15 but Dr. Center (developer of the bile acids test) says that it shouldn't be over 25. I think your pup is likely just fine.
If the number stays at 23, I don't even see why she would need a low protein diet. Bile acids can go up anytime there is a liver functioning issue, so maybe it was just a bit aggravated that day. Of course it could be MVD and rarely with a number that low it could be a shunt. Since she is asymptomatic, I would just redue the bile acids test. Based on Dr. Center's guidelines, some people may not even retest her at all. I would personally want to see it below whatever the particular lab says it should be below though.