As per documentation, for 254 characters or less, the string field is a single-line text field. Anything 255 characters or above appears as a multi-line text box.
Please note that this is expected behavior.
By default, multi-line text fields have a 4,000-character limit. To help users see how many characters remain before they reach the limit, you can enable the glide.ui.textarea.character_counter property.
Please refer to documentation for more details here and knowledge article KB0685779
