I ran into the oddest thing after switching ISPs. IPv6 kept dropping out with my devices and I traced it back to the LAN side of my router accepting router adve

I ran into the oddest thing after switching ISPs. IPv6 kept dropping out with my devices and I traced it back to the LAN side of my router accepting router advertisements from inside my network. Easy enough to fix, I flipped the flag to not accept router advertisements on the LAN interface. The weird part is that I traced the router advertisements as coming from an old Google Chromecast. It was advertising the prefixes of my old ISP. Bug or intended? If the latter, why?