I've just been playing with this and this seems to work ok for me. I could filter my dive list to find the dives I wanted to export and then export these to a .ssrf file, i could do this in a couple of batches if I wanted to filter in different ways. Then i open the five log i want to import them into and I can import from the log file. One of these imports was not showing at first but that's because my filter was still on so i removed the filter and i could see all the dives i expected. I'm doing this on Subsurface (just one capital S please) version 6.0.5299-CICD-release so a marginally older version than you but not really significantly older.
I think you are UK based like me so if i can help you do this in any way then let me know.
Jason