According to Dmitry Kobak, some details in these figures are merely convergence artifacts, and no longer produced when using more recent versions of UMAP. https://twitter.com/hippopedoid/status/1318917878364672001?l...
Example of what could cause a swirly chain in UMAP: if A related to B and B relates to C but A does not relate to C and so on. IMO that's a valid structure to visualize as a swirly chain. If re-run multiple times, of course you will get that chain in different locations and so on. But it is interesting that it is there.