Modules-->Advanced (for all HP modules as well as all warp bubble modules)

Master Hull Building unlocks Massive Scale Building as well as Reinforced Hull Design. Reinforced Hull Design then unlocks Hardened Hull Design. Upon researching Master Hull Building, I see the highlight moving to Massive Scale Building, but I can see Reinforced Hull Design in the tree as well as just above it in the list-and it's not greyed out.

Is it possible you're using a race with a tech tree that does not include Reinforced Hull Design and Hardened Hull Design?

I was assuming based on your mention of Hull Strengthening as well as the Exoskeleton Hull Point that it provides that you were using the Drengin tech tree, which does have both of the techs we're discussing, but a number of other tech trees do not: Namely the Altarian, Drath, Korx, and Thalan.

It might also help to know what version you're running-that is, if it's not the latest.