Reproduction steps:
1. Open attached project "SpriteExpression.zip"
2. In Project window, select "test.png"
3. In Inspector window, open Sprite Editor
4. In Border "L" field, after "0", type "+55"

Expected result: The field waits for the expression to be finished
Actual result: The field is evaluated immediately after typing "+5"

Reproducible with: 2018.4.23f1, 2019.3.15f1, 2020.1.0b10, 2020.2.0a13 (2D Sprite 1.0.0)

Notes:
1. Issue reproduces with the following operators: +, -, *, /, %, ^
2. Copying the remained of the expression and pasting it in works around the issue (e.g. "+13-8*2" after the value)
3. Issue does not reproduce with fields outside of the Sprite Editor (e.g. Inspector transform position X,Y,Z)