Thanks for the thorough reply! So I knew the ISP wouldn't be able to give you an IP outside of the cidr block, but was wondering why not shuffle within it.. but I understand your point about it being a pain for them to shuffle even within that block.
Some can, and do, although not usually with privacy in mind.
https://en.wikipedia.org/wiki/Carrier-grade_NAT for the main example, although even in cases where IPv4 exhaustion is not as imminent, you might be assigned a dynamic IP address. (Whether that dynamic IP address rotates on a regular basis is another question... but it's really just the ISP reserving the right to do so.)
In general, you need some stable identifier for a connection. Traditionally (namely with TCP) that's been the "5-tuple" (source IP address, dest IP address, protocol, source port, dest port). This provides network routability and dispatch to the appropriate application within a host. Your ISP can't just rotate your IP address without breaking all of your existing TCP connections.
With QUIC, that identifier is now the QUIC connection ID (although you still have the 5-tuple). Connection migration (if supported by the application) could allow you to persist a connection across lower-level protocol events (even something as drastic as switching from WiFi to 4G, where you might not even have the same ISP).
