Input text area - caret changing size on enter key


I have created a contact form field and a ‘text area’ for where users can write a message.

I have used custom code to create my fields as I wanted them to animate in a certain way. After publishing my site to the staging link I noticed an issue with the text area:

The blinking caret on the first line works great when a user types a message but when they press the ‘return key’ to start a new line, the caret size changes and all the spacing is off.

I have no idea why this is happening - it seems like a bug as nothing in the CSS should make it behave this way.

Staging site (please make your way to the contact page and review ‘Message’ input field: