Reproduction steps:
1. Open the “RectTransformPosY“ project
2. If the “BugScene” scene is not open, open it and reopen the project
3. Select the “Image“ GameObject
4. Observe the “Pos Y” value in Inspector

Expected result: “Pos Y” is 150
Actual result: “Pos Y” is 0

Reproduced in: 2021.3.28f1 (“Pos Y” is 128), 2022.3.3f1 (only in Player), 2022.3.4f1
Not reproduced in: 2021.3.27f1, 2022.3.2f1, 2023.1.3f1, 2023.2.0a22

Reproduced using: Windows 11
Not reproduced using: macOS 13.4.1 (Intel)

Notes:
1. Also reproduced in Play Mode and Player
2. Not reproduced with “Pos X”
3. Can be temporarily solved in Editor by reopening the scene