There is no option in Captivate to Save As (previous version). I know that Microsoft Office and some Adobe apps might have been able to do something like that but Captivate cannot.
You are correct that Captivate is BACKWARD compatible in that newer versions of Captivate can open older versions, but it is not FORWARD compatible. This means that if you upgrade a CPTX file by editing it with a later version of Captivate then you often run into problems because that file can then ONLY be opened and edited by that later version.
Until Captivate 2019 this only usually happened when you upgraded a project to a new major version of Captivate. E.g. going from Captivate 2017 to Cp 2019. But due to the extremely long time it has taken Adobe to produce a new major Captivate version we have seen that some of the minor version updates for Cp 2019 have also resulted in files being unable to be opened and edited by earlier Cp 2019 versions.
So, if you were to decide to unitnstall Cp 2019 version 11.8.1 and roll back to an earlier version, I think you would probably find that you can only roll back to 11.8 and not earlier, otherwise you would lose the ability to edit your project.
A more productive endeavour might be to try and resolve the issues you are having with 11.8.1. What exactly are the bugs and problems?