Are you using Tableau Server version 9.3.3? This issue is supposed to be resolved in that build.
If not, please upgrade Tableau Server and test the export again.
If you are using 9.3.3 already, please file this issue with our technical support team.
Thanks for the reply! No, I haven't upgraded to 9.3.3 yet, because I was exporting the sites in preparation of moving our data centers, and we are not allowed to install or upgrade anything on the servers until then. But yes, we will install 9.3.3 once the move is complete, and then I will try again.