I feel a line of an article somewhat strange gramatically

Question

I am currently reading this article, which has the below line,

The outcome also reflects the seismic effect the U.K.'s decision three years ago to leave the 28-nation EU has had on the country's politics, with voters increasingly split into pro-Brexit and pro-EU camps.

I know the "construction" of this bold line, but wouldn't it be a bit strange?

"The outcome" (Subject) is "reflecting" (Verb) the seismic effect (the direct object), the U.K.'s decision three years ago to leave the 28-nation EU has had on the country's politics, (modifier line to the word 'effect'), but wouldn't this line be lacking the probable possession particle "of"?

If I or we insert "that" between the 2 words, which are effect, and the U.K, it would sound more strange because the potential "that clause" is ending with the noun, which is "politics", which in turn means the modifier line is unfortunately (probably) is completed as one-set line perfectly.

Am I still wrong here?

I am afraid to say that there are many that "that" is implied here or works, but personally, how many times I read the line in the question adding "that", anything does not make sense at all, and personal possible option "of which" is also complicating.

Answer 1

The sentence you quote was not written by anybody who was trying to be clear. But apart from lack of clarity, it is grammatically OK.

I think that you are understandably being confused by the very complicated noun-phrase at the end of this sentence. In outline it says:'The outcome (also) reflects [something]'. Something = 'the effect [x] has had on the country's politics', and some details fo that effect are then described.