Transition Protection

That is correct. Any increases in other elements are taken off the transitional amount. In effect people with a transitional amount have their UC capped at a fixed amount until such time as the transitional amount is fully eroded (which could be several/many years.

People who were transferred from IB to ESA were treated in the same way.

Not saying it is fair though.

janer1967 said:

It doesn't seem fair but try and think of it the other way that while you have have the transitional payment in place you are getting more than those who are new claimants 

Yes but new claimants will not have been previously claiming the SDP in another benefit, therefore they wouldn't really be missing out on anything. Even with the SDP transitional payment a claimant can still be worse off by claiming UC.

https://policyinpractice.co.uk/what-end-of-sdp-gateway-means-for-disabled-people-moving-to-universal-credit/

3. Households whose transitional payments will erode quickly

The SDP transitional element is a measure introduced by the government to soften the blow when people move from legacy benefits onto Universal Credit. It is designed to decrease over time, so that in the long run, no one will be receiving SDP in working-age legacy benefits or a SDP transitional element. If your circumstances change and any element of your Universal Credit (apart from the childcare costs element) increases, your transitional element will decrease by the same amount. For some households, this erosion will happen quickly. 

There are examples of how quickly the value of Transitional Protection can decrease, (within 2 years).

As it says, No one will be receiving SDP in working-age legacy benefits or a SDP transitional element in the long run, however long that takes.

A very explanatory read.

Edit -  Lets not forget the EDP

2. Households who lose other elements of their legacy benefits when they move to Universal Credit

The transitional element of Universal Credit only compensates for the loss of SDP. This means that people can still lose hundreds of pounds a month when they move to Universal Credit depending on their circumstances. Indeed, disabled people in receipt of the Enhanced Disability Premium (EDP) have never benefited from the SDP gateway and have already been moving onto Universal Credit. They tend to receive less money under Universal Credit since there is no equivalent for the EDP within Universal Credit, and they do not receive any transitional payments.

Whilst the idea that someone isn't going to miss what they never had, it's still affecting some of the most vulnerable people in society, and that to me, is unacceptable.