The runtime dependency on flatpak-session-helper (ie., Requires: flatpak-session-helper) was mistakenly removed, on the assumption that it's only needed to use containers created by Toolbx < 0.0.97.

Unfortunately, that's not true. It's needed on the host when toolbox(1) invocations inside a container are forwarded to the host with flatpak-spawn --host .... This has been true since Toolbx >= 0.0.6, and, hence, flatpak-session-helper is needed on RHEL.