Unfortunately the official TWRP app also doesn’t seem work for A/B devices (yet).
But there’s a Magisk module: TWRP A/B Retention Script (twrp-keep)
I’ll give it a try at the next eOS update.
https://github.com/Magisk-Modules-Repo/twrp-keep
https://forum.xda-developers.com/showthread.php?p=77770683#post77770683