This is an issue I am having as well in the SSIS package. Apparently, they were supposed to be fixing it as of the next release, but that came out nearly 2 weeks ago and I tested again, only to see it behave the same way. I was told that the workaround would be to change the TargetServerVersion property to in the Project > Properties menu to SQL Server 2017, make your changes in the Analysis Service Processing task, then change back to whatever version you were using before deploying. Am I correct that your existing package is targeting SQL Server 2016?
Also, if you are not having this issue any further, I'd love to hear how you were able to fix it. I even tried the "workaround" found here, but that didn't work for me either.
Michael Mallit from MSFT said on May 16th, 2018, "This is the believed fix that we have on our side that will be in the next release."
It is now the next release and I don't see this fixed personally