There's a new build imminent, so I've taken the liberty of sending in a possible bug report based on what I've read in this thread. Manifold has often done tweaks in one day, so why not go for it?
That's not as good as a bug report from somebody actually working with PostgreSQL 13 (I still use 11) sending in full info, but the sooner the better for these things. See, for example, https://georeference.org/forum/t147098.3#147104
Don't forget to include all details, like which DLL you're using for the PostgreSQL client, the queries used, etc. All that stuff is important because Manifold handles materialized views in a more sophisticated way than simpler packages. See, for example, the discussion in threads like https://georeference.org/forum/t142625#142678 and https://georeference.org/forum/t157665.10#157741