フラれる can be written like this, but yeah it means to get rejected or dumped or things like that. I would translate it as “I thought he would come today, but it looks like he ditched me.” The reason the machine translators are introducing things like “I guess” is because you can translate みたい in different ways. “It looks like” matches the Japanese wording the most, but it’s not necessarily the best way to translate it in every context.
Also I don’t think DeepL is better, I personally don’t think machine translation tools are very useful outside of using them for entirely unknown languages. In that case, it’s “better than nothing” as it were. My view is a bit extreme so feel free to disagree with it; I know some people find it useful for getting a gist. I often find what it spits out to be more confusing than helpful, and even when it doesn’t make any mistakes, it incentivizes me to give up on sentences and rely on English instead of trying to understand them in Japanese.
This happened because using the kanji gives the impression that you intend the literal meaning of “shake” instead of the metaphorical meaning. A native speaker would probably understand from context what you meant, but the machine translators probably rely on whether something is in kanji or not to help them try and guess the context. I think that writing it in katakana here makes it more clear that it’s being used in its figurative meaning of “getting dumped/ditched/rejected.”