I think the release notes had something to say about that.

There was no release note or other document on the download package

Have a look here

https://www.sagecity.com/support_communities/sage300_erp/f/sage-300-announcements/159797/sage-300-2021-product-update-1-available-la-mise-a-jour-du-produit-1-de-sage-300-2021-est-disponible

Thank you for provided links where it is informed that after PU1 the editing option is not available, but previously customized process flows should work. We really found one of our customized process flow that works; however, others are not opened at all.