I hadn’t noticed it (I don’t use SFTP generally), but I can reproduce it. Also confirming 2.0.4.5 doesn’t do it.
C:\>netstat -ano | findstr :22 | findstr ESTABLISHED
TCP [::1]:22 [::1]:65411 ESTABLISHED 3584
TCP [::1]:65411 [::1]:22 ESTABLISHED 18476
is a way to count connections (if going to localhost as above, expect connections to show mirror-images). Limited tests did seem to show it as upload related, e.g. test of downloading all files had no open leftovers.
This seems like a serious limit, correct? If (not confirmed) it was in v2.0.4.16-2.0.4.16_canary_2019-03-28 then I guess nobody picked up canary and noticed it. There’s a wish-it-was-seen-sooner backend-specific bug in “FTP (Alternative)” in Problems after upgrade to 2.0.4.21 which requires a UNIX-style FTP server…
Although I hope SFTP can be fixed, there’s some time pressure from Amazon Cloud Drive access ending. There are other SFTP servers, but free ones sometimes have limits like yours, or only allow personal use.
Characterizing the problem further will help figure out where it lies. Even better would be if you read code…