Why can’t I select individual letters in the ‘ffi’ of ‘difficult’? [on hold]

In some instances, I can select any part of the word 'difficult' freely; textboxes and the post preview are two of these places:

However, when attempting the same selection in other spaces; for example, Slack messages, or the Google search result for 'difficult' I find myself unable to select any smaller part of the 'ffi' individually:

What is the cause of this?