This is a challenging bug report.
I have been using After Effects since 1995 when it was owned by CoSa. I have never had a problem like this. Strange After Effects issue: Both of these movies used the same After Effects project file. The opacity fade didn't work in CC2018. After two hours of testing I rendered one last test in CC2018 (see attached file) and was not able to see the simple opacity fade. I then save the AE project file back one version to CC2017. I rendered the same test without changing a thing and was successfully able to get the simple fade to work. I am using a late 2013 Mac Pro with 32 gigs of RAM and OS 10.11.6
Any ideas on how to solve this issue would be approeciated.

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Roei Tzoref commented
posting the project files would help
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Anthony Venezia commented
Sorry about the typos.