Steps to reproduce:

Open the attached project
Run QA > 1. Author. It puts a Ratio of 1.5 on a MonoBehaviour, saves it to a prefab, and prints which fields the Inspector shows and which reached the file
Close the Editor and reopen the project
Run QA > 2. Verify (run after a restart)

Actual results: Step 2 reports that "ratio" is a visible Inspector property, so a user can author into it, and that the saved prefab contains no "ratio" entry at all. Step 4 prints "VERDICT: FAIL - ratio silently lost": the field comes back 0 instead of 1.5. Nothing is logged at any point. The two control fields on the same component, a float and a Vector2, both survive. Pressing Play has the same effect and does not undo it, because Play mode serializes and restores the scene and there is nothing to restore, so the authored value is destroyed rather than suspended.

Expected results: Either the value persists, or the field is not offered in the Inspector. UnityEngine.UIElements.Ratio, AnimationIterationCount and GridLine are marked [Serializable] but declared as readonly structs with readonly backing fields, and Unity's serializer does not write readonly fields, so [Serializable] buys a SerializedProperty and an Inspector row but no persistence. The sibling types added alongside them, Curvature and GridTrackSize, are [Serializable] and NOT readonly, and they round-trip correctly. Ratio.Auto() is the worst case, because it is encoded as NaN: reloading does not merely zero the number, it destroys the sentinel, and IsAuto() flips from True to False. USS and inline C# styling are unaffected, so the defect is confined to these types used as serialized fields.

Reproducible with versions: 6000.6.3f1, 6000.7.0b2, 7000.0.0a1

Not reproducible with versions: 6000.3.24f1 (Ratio exists but is not [Serializable], so no Inspector row is offered and nothing can be authored into it)

Can't test with versions: 6000.0.84f1 (UnityEngine.UIElements.Ratio does not exist)

Tested on (OS): macOS 26.6.2 (Darwin 25.6.0), Apple silicon