Hi @nikviz,
What I can see in the Adaptivecards designer on https://adaptivecards.io/designer is that when you make a Input.Text control required you would also need label and errorMessage properties in your json.

Can you try something like below and see if that makes a difference?
{
"type": "Input.Text",
"placeholder": "Please enter your Name",
"label": "Name",
"id": "Name",
"isRequired": true,
"errorMessage": "Required input"
}
Also tested it with the json below.
{
"$schema": "http://adaptivecards.io/schemas/adaptive-card.json",
"type": "AdaptiveCard",
"version": "1.3",
"body": [
{
"type": "TextBlock",
"text": "Default text input"
},
{
"type": "Input.Text",
"id": "defaultInputId",
"label": "Comment",
"errorMessage": "This is not valid",
"isRequired": true,
"placeholder": "enter comment",
"maxLength": 500
},
{
"type": "TextBlock",
"text": "Multiline text input"
},
{
"type": "Input.Text",
"id": "multilineInputId",
"placeholder": "enter comment",
"maxLength": 500,
"isMultiline": true
},
{
"type": "TextBlock",
"text": "Pre-filled value"
},
{
"type": "Input.Text",
"id": "prefilledInputId",
"placeholder": "enter comment",
"maxLength": 500,
"isMultiline": true,
"value": "This value was pre-filled"
}
],
"actions": [
{
"type": "Action.Submit",
"title": "OK"
}
]
}
