Steps to reproduce:

Create a new Unity project using the Universal 3D template
Create a TMP Input Field (GameObject > UI (Canvas) > Input Field - TextMeshPro) and click "Import TMP Essentials" in the TMP Importer window
Import the attached "STIXGeneral.otf" font into the Assets folder
Right-click "STIXGeneral.otf" in the Project window and select Create > TextMeshPro > Font Asset > SDF
Select the "STIXGeneral SDF" asset, open the Inspector's burger button menu and click "Import Font Features"
Select the "InputField (TMP)" GameObject in the Hierarchy and set "Font Asset" to "STIXGeneral SDF" in the Inspector window
Select the "Text Area/Text" child GameObject and enable "liga" in Font Features under the Extra Settings section
Enter Play mode
Click the Input Field in the Game view and type "offi"
Press the "Backspace" key once
Observe the Input Field text

Actual results: The text becomes "o" (all 3 letters of the "ffi" ligature are deleted)

Expected results: The text becomes "off" (only the last letter is deleted)

Reproducible with versions: 6000.6.5f1, 6000.7.0b3, 7000.0.0a5

Not reproducible with versions: 6000.0.84f1, 6000.3.26f1, 6000.6.4f1

Tested on (OS): MacOS Silicon Tahoe 26.6.2

Notes:

Doesn't reproduce when "liga" is disabled in Font Features (Backspace deletes only the last letter)