The tree is declared as a "Citrus Ichangensis x Citrus reticula" but the buldge looks suspiciously like something after a grafting process.
I don't have a problem with it but the seller didn't mention any grafting being done.
I am also cautious because I live in a zone 7a area and don't wanna kill my first attempt.
There does look like a discontinuity around that entire perimeter but the bark structure looks like the same species. I don't know why you would want to graft to a same/similar species. Could always ask.
Btw the links are hidden in the post description. Only showed up when I went to comment.
Almost all commercially sold citrus is grafted. Citrus is generally not true to seed, so a known good fruit branch will usually be grafted onto a seed grown citrus species that is specifically grown to be grafted onto. This not only ensures that the fruit is produced as advertised, but also helps the plant reach fruit production more quickly.
Disclaimer: I'm just some guy with a few fruit trees, definitely not an expert.
So it's kinda as assumed in my comment and as OP has identified?
Yuzu that tastes good was grafted onto another yuzu that maybe was declared as tasting not as good?
Or is this not a thing and it's something like a trifoliate graft?
Like I mentioned: The nursery does not mention grafting.
According to EU-law grafted plants need to be marked as grafts on their passport.