Hmm, if you do a local update from system settings it will be installed in the other slot. So the partitions will be overwritten.I have the same problem as yours. I've tried countless scenarios, followed tutorials on XDA and Youtoube but can't make TWRP work.
Out of curiosity I restarted the device through slot A after flash zip from magisk. My phone has accessed TWRP, but I cannot access my files. When I restart from slot B I have the opposite. Twrp doesn't work, but my files are accessible.
So I believe I somehow "broke" slot A. I just don't know how I did it or how to fix it.