So, a peculiar thing. I'm using Rich Skryrim Merchants, https://rd.nexusmods.com/skyrimspecialedition/mods/1772, using the "RichMerchantsSkyrim_x5.esp, with perks" variant, and it contains vendor leveled item updates. WB thinks the plugin is mergable, but when I look at the patch afterwards while it contains appropriate vendor records, the count values are what is found in Skyrim.esp. It's as though WB has generated new records but dropped the actual values it needs to update the records with on the floor.
What's more interesting is that if I mark RichMerhantsSkyrim_x5.esp as not mergable, then also contains updated vendor leveled item records, but again these contain only vanilla values, not the values from the plugin. This means that I can only have this plugin take effect if I move it after the bashed patch.
Is this a known thing? I'm using "Wyre Smash 307.201801021732", standalone executable build.
Update: Fixed it. When I looked at the plugin with LOOT it had a "Relev" tag attached to it, but this was not appearing in WB itself. When I manually added it in WB it started behaving itself properly.