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I ignore these links when I do backlink audits.  They're obviously not self made and that's what matters.

If your existing subdomain is ranking and earning well, it is a risk to move that to a new domain, because not all authority/history/etc. is transferred and you have to be very careful about not changing anything else and still have to hope that all works out as expected. Building links and authority to the new domain and then linking to the existing subdomain has basically no risk to your current business. Only problem is that you will have to create unique content and get new links, but at least there is no chance of hurting your existing rankings and the real possibility that after the 301 the subdomain is stronger. But the above is short term thinking, for long term it could very well be wiser to bite the bullet and 301 the old to the new. But that all depends on what you want for the future. ***** Extra info: In the past 8 months I have acquired 10 EMD and 301'd old domains to those new EMD's. 5 websites gained ranking, 3 more or less stayed the same and 2 disappeared to 200+, from top 10. All this was done to websites 4 years old with stable rankings for 2 years and no SEO/link building done for 2 years. So for me it really was what I felt the luck of the draw what happened with 301's. Later I found that it is not only luck, it also has to do with why the old website was ranking in the first place, content or links was most significant. This of course besides the fact that you should leave everything and I mean everything the same when 301'ing.