If post transcriptional modification happens , how can a gene code for amino acids in a particular order?
"then shouldn't the protein produced be different every time? Shouldn't the order of amino acids be different? And if it is, doesn't that mean that the protein produced is different?"

It is not different every single time, but it is different depending on what kind of post transcriptional modification takes place. The proteome is larger than the genome thanks to this, genes can code for many different proteins thanks to post transcriptional and post translational modification as well.

So as I understand it, gene segment = same order of nucleotides = same primary mrna

post transcriptional modification to primary mrna = different mature mrna = different order of amino acids = different polypeptide chain (then different protein later on)

Hope this helps