Sometimes the lead connecting the wandering trader to its llamas "detatches" from the trader and appears to get "stuck" at one point. The llamas still follow the trader. The leads will appear to render from wherever the trader was, to the llamas. The lead can go through blocks and stretch fairly far distances (~45 blocks+).
If I'd take a stab at this, I'd guess that the lead updates the position of the mobs involved every so often, and then sometimes the position the trader is at gets "stuck" to the lead, so it stays in one spot.