I have the masonry grid option enabled and published but the widget still maintains a fixed height grid. I have tried using an unmodified post grid to see if my customizations had caused this and there was no change in behavior. Here is an example: https://imgur.com/a/qN26DUt
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Have you set grid item height, that might cause the issue, please try
Grid Items Height > Auto height
Please see the screenshot
If possible please send me your post grid page url.
I'm just silly and still has some custom css in from some tinkering. For some reason
absolutely breaks the feature. Curious as to why although it is no longer a problem.