User becomes unsubscribed on his topic after editing his first post in this topic

Environment: 2.14.2769.dbed6b5 (preprod)
3.0.2783.8629f39 (QA)

Steps to reproduce:
1. Open "Notifications" in User's Profile page and uncheck "Auto subscribe"
2. Go to Branch, unsubscribe on this Branch and create a Topic in this Branch
3. Open Topic and subscribe on it
4. Click on the button "Edit" in the first post and then (without any changing) click on the button "Save"

Actual result: Topic opens and User becomes unsubscribet on it (link "Unsubscribe" changed to "Subscribe"
Expexted result: Topic open and User is still subscribed on Topic

Note: This problem also reproducible in Q&A topic type