deterministic uuidv4 -> uuidv4
Java's internal secure random generator works along similar principles, just uses the older SHA1. By rehashing a starting point (seed), you create a ersatz cryptographically secure pseudo random number generator (CSPRNG).  Albeit, you're only creating one output per seed.  

The entropy isn't really affected and truncation is okay as we consider all of the SHA256 output bits as independent.  Unless you have very particular requirements and used a true random number generator, the likelihood is that the UUIDs were originally generated by some form of CSPRNG from the underlying OS.  You're just adding another round of processing.

The number of random  UUIDs (of 122 random bits) which need to be generated in order to have a 50% probability of at least one collision is $\approx 2^{\frac{n}{2}}$ or $\approx 2 \times 10^{18}$. There's a little bit more in the wiki article.