Four bugs here
First, ZQuest seems to leave 0: (none) strings behind when deleting the default quest strings. Currently, I have 8. This leads into the next bug
Deleting all of the strings in a quest results in a lot of these 0: (none) strings, however it also somehow removes the <New> option at the bottom of the list
When reverting to the previous save of a quest, and you've selected one of these 0: (none) strings, it will overwrite the string that was there in the original save. For instance, deleting all the strings then selecting the second from the top (1: IT'S DANGEROUS TO GO ALONE! TAKE THIS!), then reverting to the old save will generate a 0: (none) string there, and the next string will be 2: BUY SOMETHIN' WILL YA
Finally, and this is probably related to the glitched 0: (none) strings, but ZQuest doesn't make the default 'Next string:' to 0: (none). Instead, it defaults it to the previous string edited, and since 0: (none) strings can't be edited... yeah
[Edit] Seems ZQuest fixes some of these issues itself. When you re-load (Not Revert), it sees these 0: (none) strings as corrupted strings, and reverts them to the quest's default strings. It does preserve your custom strings, as well. However this brings a new bug, or clarification as to the second I posted earlier
Deleting a default string will also transform the <New String> into a 0: (none). However, re-loading your quest also fixes this
You can save your quest with these issues, and re-load it, and ZQuest will fix them. You can then delete the default strings again (Probably just a few at a time), rine and repeat, and eventually, these bugs will fix themselves. All of them, it seems. But I don't think these should even be happening to begin with