This only happens if the question index is set to full.

First impression:

var_dumping the session variable shows that the values of the previous questions are correctly saved (bot in question index full or disabled).

In SurveyRuntimeHelper::createFullQuestionIndexByGroup(), this snippet cause the bug:

$stepInfo = LimeExpressionManager::singleton()->_ValidateGroup($key);

after further investigation, the problem is deeper than it seemed:
Deactivating the question index partially solve the problem.

Even with no index:

1. Answer A to the first question on first page
2. Click next
3. In second page, A is shown (ok)
4. Click prev
5. Change your answer from A to B
6. Click next
7. A is still shown as the answer (wrong, should be B).

Maybe we should document about it, saying that for now, using expression should not be done in survey allowing "Previous" or "Index"

after a discussion with Carsten, it seems that the wanted behavior (default values is used only the first time)

So, here my bet:

the function _ValidateGroup is called at the very start of the survey, to build the menu. Doing it, it's saving the (empty) values of the answer to session, overriding the default values.

Possible solution:
Only apply _ValidateGroup to the previous group.

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=20241

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=20264

Version 2.51.0 Build 160829 released