I found, looking at post Penpot behind a reverse proxy - websockets, that I need to get web sockets involved in the proxying operation. The exact configuration from that post didn’t seem to work, but there were a couple different ways it seemed to work. I settled on keeping the ProxyPass and ProxyPassReverse that had been sort-of-working, and adding the rewrite rules listed (four lines of RewriteXXX directives).
