I have the same problem.
When the second and subsequent posts, the featured image is affected by the size limit of the image field.
Environment
– WordPress 5.4.2
– Advanced Custom Fields PRO 5.9.0
– macOS Mojave 10.14.6
– MAMP PRO 5.7
– Firefox
I’ve seen it reproduce in Chrome and Safari as well.
Prerequisites
– image field: Minimum width: 300px, height: 300px, Maximum width: 300px, height: 300px
– The featured image I want to register is a different size. This time 1200px * 800px.
– Added 2 images to Media Library: 300px * 300px, 1200px * 800px
Point!
The bug will not be reproduced the first time you register a post.
Steps to reproduce
1-1. Create 1st post
1-2. Add an image to custom field
1-3. Add a featured image
1-4. Click publish button
We won’t reproduce it here.
2-1. Create 2nd post
2-2. Add an image to custom field
2-3. Click to add a featured image, 1200px * 800px, you’ll find the message: “Restricted 1200_800.png
Image width must not exceed 300px.
Image height must not exceed 300px.”
3-1. Create 3rd post
3-2 Add a featured image
3-3 Click to add an image to custom field, you’ll find No size limit
I uploaded video to YouTube.
Please let me know the solution for this.
Thank you.
