I have had people fill in my words but I don’t believe I’ve ever been perceived as being rude because of it. In a sense you are a bit fortunate because “thank you” is easily something you can practice because there are so many opportunities to say it. I would suggest using your thankfulness for whatever they did as positive energy to help you say the words “thank you”. You can even smile while saying thank you, then if you start stuttering, at least the person will feel at ease by you smiling. But it must be difficult to think about how that person may perceive you. But if you see them again, just tell them you wanted to say thank you but that you stutter sometimes