Weirdly enough… I’ve seen some Japanese people here use the full kanji for 有難う御座います a handful of times (I work at in a Japanese company).
My guesses as to why would be:
- Their IME switched it to the kanji and they didn’t realize before pressing send.
- It was a specifically formal moment and they wanted to add an extra layer of formality.
Same goes for stuff like ラーメン → 拉麺、コーヒー → 珈琲, etc. (which are actually used in kanji more often than ありがとう I’ve seen).
The point being, use whatever kanji you like!!! Just like some people have different speaking styles, there are definitely different kanji-use styles. Go with whatever makes you happiest ~