To be honest, your builder was probably just slack and lacking attention to detail.
To say he intended to de fr a ud you is actually pretty defamatory and more then likely not the case. (I know you didn't claim this)
Chances are he didn't even look at the bracing plan when Quoting (like most won't) and forwarded it straight to his supplier of the frames. The frame supplier then came up with a certified design using their software and the quote was based on that.
The builder then got the price back, put it together with his roof quote which also came back from the roofing company and so on until he arrived at his price.
You then accepted and then he processed his purchase orders based on his supplier quotes.
That is the likely situation.
If a question was raised by the supplier to the builder about the power braces it would have likely gone like this........
We notice the bracing design calls for power braces.
The builder would have said what's that, the supplier would have said its a propriety brace. The supplier would have said when we did the design we came up with another option and the builder would have been like yeah whatever that will do as long as it works.
The missing step is the builder should have taken it to you and said we have an alternate solution to these braces and so on.
But when the builder prices 50 homes a week and gets 3 I'm sure he isn't going to fuss about the specifics too much
Is the build "sub standard"? Likely not
Just to reiterate, I do feel he should have taken the omission of them to you.
This part was just plain slack.
As for your original question.... do framers change bracing plans often......no, not usually because it takes them time to do so unless there is something that doesn't work properly. Do framers change any other specs of the frame? Yes, all the time.
Plans often state for frames and trusses to be supplied to suppliers engineering (or words to that effect).
This is why they have to provide a certificate upon delivery