I’m not sure how common it is but I have the same issues with my vintage pendant! It annoys me to the point I don’t wear it as much I I would like so if anyone has any bright ideas on avoiding it happening I’d appreciate them too!
I have 5 vintage pendant's and I've never seen this issue. I do have 2 that are 18 inches long so when I wear it at the shortest setting, the extra length of the chain gets twisted a bit with the rest of the necklace. It's not really that bothersome for me...I just untangle.