Strange bug when using Fluid Field

It seems like Fluid fieldtype is buggy with EE 6.3.5. My english is not so good, but I will try to explain.

The bug happens when I do this:

I create two fieldtypes, a grid named “Title Content” and a filegrid named “Image with caption”. “Title Content” has two Grid Fields: “Title” (Textfield) and “Size” (Radio buttons). “Image with caption” has one Grid Field named “Caption” (Textarea). The fieldtypes are then added to the fluid field named “Fluidcontent”.

I make a new entry and add content by using “Title content” og “Image with caption” in the Fluid-field. (See screenshot #1}

No problems yet.

Then I add another “Title Content” in the fluid field. (See screenshot #2)

Still no problems.

But when I now tries to add another picture in the already existing “Image with caption” field, I get a strange error. No other explanation than “Cannot Update Entry, please review and fix errors”. A red box around the radio buttons in the “Title Content” is present. (See screenshot #3)

I’ve tried to remove the grid field with radio buttons, but the bug appears non the less.