Relays in the Tor network have varying CPU and networking performance. This means that the throughput of a circuit depends on the performance of the relays in the circuit. Some circuits will be slower and others will be faster. Having a video that loads quickly does not indicate any kind of attack, and you just may be accessing the website over a circuit ...
I've messaged one of the tor developpers and he's been kind enough to answer. In the rend-spec-v3.txt document, the function is actually specified in appendix A2. And the "HSDirV3" flag never existed. It'll be removed from the specification.
So it seems the public key is only 32 bytes long, but in your example it is 64 bytes long.
I coded the following script, given the full public key of 64 bytes and got the sample address you put. So take it as a reference!
from base64 import b32encode
from hashlib import sha3_256
# Expected result: ...