I am able to replicate this issue also. The problem presented itself after the installation of the Purity IV and T4 BS5 templates. PB was working as expected with Cassiopeia template, breaks on template change to Putity IV or T4 BS5, then works as expected again when switching the template back to Cassiopeia...
As I was writing this, I installed the Helix framework on the site, and found I could edit the PB Pages while using it. Looks like the workaround until someone corrects this issue permanently, is to assign the Cassiopeia or Helix template to the page you're editing, make your edits and then switch back to your standard front end template, once the page is ready to publish. Probably easiest to keep track of them in your site menus, but ymmv...
Can we please just go back to editing the pages in the back end, as we could previously? Having the editor work with the Atum Admin template (which very few users ever change) is probably easier than having the front end editor be compatible with the myriad of front end template frameworks/styles available. The opportunity for conflicts with different versions of bootstrap, other additional css, and/or js seem potentially endless. Just my 2cp,
Thanks!
Fresh install of J 4.3.1
pHp 8.1
Rochen hosting