Steps to reproduce:

Open the attached "TextFreeze" project
Open the "SampleScene"
From the Project window, select the "CalibriBold SDF.asset"
In the Inspector, find Generation Settings > Atlas Population Mode property
Change it from "Static (Obsolete)" to "Dynamic"
Observe the Editor

Alternative reproduction steps (from fresh project):

Create a new 3D URP project
Import the attached "CalibriBold.ttf" font and select it
Go to Window > Text > Font Asset Creator
Attach the imported font as "Source Font" then Generate Font Asset and save it to project
In the Hierarchy, right-click > UI Toolkit > Panel Renderer
In the Inspector, click "New" and open it in UI Builder
Add a label to the Hierarchy
In the Inspector, expand the Text foldout and assign the created Font Asset into the Font property
Save and exit UI Builder (observe errors appearing Console)
Select the created Font Asset from the Project window
In the Inspector, change its "Atlas Population Mode" from Static to Dynamic
Observe the Editor

Actual results: Editor freezes and "Hold on" popup appears, Unity rapidly consumes all available disk space and process needs to be killed from Task Manager

Expected results: The Atlas Population Mode changes without freezing the Editor or abnormally consuming disk space

Reproducible with versions: 6000.5.0a9, 6000.5.0f1, 6000.6.0a7

Not reproducible with versions: 6000.4.12f1, 6000.5.0a8

Tested on (OS): Windows 11

Notes:

Reproduces with both TextCore and TMP
Does not reproduce on macOS 26.5
The Font Asset must be referenced by a UI Toolkit element to reproduce - i.e. assigned to a Label in a UXML that's used by a Panel Renderer / UIDocument (as set up in UI Builder). It does not reproduce if the Font Asset is not referenced/in use anywhere
Disk space is reclaimed after the process is killed but the drive fills completely during the freeze, risking failed writes, project/data corruption, and loss of unsaved work, plus OS-wide instability while full