Existing field handle overwritten upon edit

[lhs-dev]

Description

When I edit an existing field, its handle is overwritten with the slugified version of its title.

Steps to reproduce

1. From the field manager, edit an existing field (field name: Content builders - Pages de contenu, handle: contentBuildersMain)
2. When the edit modal has finished loading, the correct handle is displayed (contentBuildersMain)
3. [Optional] Make some edits
4. Click the save button
5. While the field is saving, before the modal window is closed, notice the handle has changed (contentBuildersPagesDeContenu)

Additional info

• Plugin version: 2.1.8
• Craft version: 3.4.x and 3.5.x
• Multi-site: Yes

Additional context

This happens every time, except when I make some changes to the handle myself:

1. From the field manager, edit an existing field (field name: Content builders - Pages de contenu, handle: contentBuildersMain)
2. When the edit modal has finished loading, the correct handle is displayed (contentBuildersMain)
3. Change the handle to contentBuildersMainNew
4. Click the save button
5. While the field is saving, before the modal window is closed, notice the handle hasn't changed (still contentBuildersMainNew)

[engram-design]

Pretty sure this was done on purpose at the time, but it's confusing behaviour. I suppose I was trying to make it easy to manage field handles.

Updated in 2.2.0