tonight I made a second bug report about this same issue because I had more information to add to the report and I didn't know how, or if, I could add it to the initial report. Unfortunately it seems to have been flagged and deleted as a duplicate report. But I also made a post on the Steam forums that included the info I wanted to add, so I'm just going to copy/paste it here:

In T179284#2570940, @speedweasel wrote:

Yet, somehow, upon double-clicking RMB with the "Raise modifier" being the only function bound to it, ADS would activate for a split second and then aim would be dropped entirely

This occurs when 'Raise Modifier' and 'Eye Focus' are bound to the same key. When your weapon is raised (in hip fire stance), the Eye Focus function seems switch you between hip fire and ADS - possibly an undocumented feature. In the case above it's happening for a split second due to the double-click.