When switching issue types, it will replace text you have already typed — but only up to a point. This page explains exactly where that point is.
The app tries to not interfere once you have clearly started writing. Therefore, templates might not be applied when changing issue type after this point.
Overwrite Rules
There are two checks that decide whether to replace user text or not:
1. The description check — 200 characters
When you change an issue type (work item type), if you have added more than 200 characters of your own, the template is skipped entirely.
At 200 characters or fewer, it continues to the second check.
2. The summary check — 5 characters
If you have changed 5 or more characters in it, your summary is kept and the template's summary is ignored — but every other field is still filled in.
Under 5 characters, the template's summary replaces what is there.