Custom notification sound for particular Hangout keeps reverting

Question

On my stock, unrooted Samsung Galaxy S3 running Android 4.4.2, with Hangouts version 6.0.107278502 set as my default SMS app, I have the following settings (only giving you those I deem potentially relevant):

General Settings

• Enable merged conversations: yes
• SMS
  • SMS account: (my gmail)
  • Send SMS from: Carrier number
  • SMS sound: Flowers

Account Settings

Notifications

• Hangouts messages & invites
  • Notifications: yes
  • Sound: Hangouts Message
  • Vibrate: yes
• Google Voice SMS
  • Sound: Hangouts Message
  • Vibrate: yes

Additionally, I have set custom sounds under People & options for three conversations. One is for 40404, the short-code for texts from Twitter; which is set to:

• Chat message sound: Chirps

This is so that I can ignore that particular notification if I'm even remotely busy, since Twitter messages are generally less important than a text from a friend.

Another is for another short-code, this one for deals from a local pizza place:

• Chat message sound: Good news

The only other custom sound I have set is for my wife, so I can make sure I always read her messages, even I'm in an important meeting; her message could be even more important. For her, I chose the sound I found to be easiest to hear when the phone is in my pocket, yet not an annoying sound to hear often if I should receive many messages in a short timespan:

• Chat message sound: Pure bell

While this conversation history does include some SMS (and possibly also MMS) messages, our standard practice is to use Hangouts messages between each other.

The problem is, every so often (on average, two or three times per week), my wife's notification sound spontaneously changes to Chirps, causing me to miss her messages because I think "that's just Twitter". If I open up the Hangout and go into People & options, the sound setting has been changed. I change it back, and it takes effect, but eventually it reverts back to Chirps again.

One time I actually "observed" the change as it happened: I was actively using the Hangouts app, and had exchanged messages with my wife within the last few minutes. As I was looking at my conversation list, the screen blanked, and then the splash screen appeared, along with a toast notification "Signed in as d***@gmail.com" - apparently Hangouts had crashed and restarted itself. When the next message arrived from my wife a few moments later, it played the Chirps sound.
The change coincided with Hangouts experiencing some kind of crash! _{On that one occasion, anyway.}

My question is:
Why is this happening, why only for this one Hangout and not for any others, why does it always change to Chirps (i.e., why not the default sound of Flowers or Hangouts message), and, most importantly, how can I prevent it from continuing to happen?

_{I'll accept any answer that fulfills the last part of the question; the rest would be interesting information, but should be considered optional.}