You really need to make a claim under the warranty. It would be in the warranty provider's interests to ensure that any remedial works are carried out to manufacturer's recommendations, design and guidance.
There will always be an allowable deflection in the design of floor joists. When this is exceeded, the design will need to be beefed up.
As you notice the deflection, I would guess it is excessive for one of the reasons I have already suggested.
As for whether bodging the problem with stiffening being common, I could not possibly know. All I know is in this one instance that was brought to my attention it was attempted and did not work.
Write to the housebuilder and copy the letter to the warranty provider.
In your nine posts regarding your floor joists, you have not once mentioned the name of the housebuilder so others are warned. This is the whole point of this forum.
As you are shared ownership, you should also involve the HA.
I have now said all I can on this matter.