There was indeed a problem with this, but it has been solved a while ago.
Everything I found was two entries in the Security Center (2019-05 and 2017-05). So the best solution would be to upgrade to a recent version (9.4.4), if possible.
Issue 2017-05 has been fixed for version 7.1.2 and higher by a patch provided. See here for more information. This has been finally fixed in DNN 09.00.02.
Changing the way the UserId is seeded or using a GUID is not a recommended way, that goes deeply into the core of the system.