After you restart Mash, 'Refresh Data' (Installers), or perform a 'Full Refresh' (Installers), the Installers tab detects the core ESMs (Morrowind.esm, Tribunal.esm, and Bloodmoon.esm) and their BSAs as missing, if they're part of a project/package and that project/package is installed.
I created a TES3 package with Morrowind, Tribunal, and Bloodmoon as sub-packages, with the base contents of the 'Data Files' directory after installing the game and its EPs. This package is for full install/uninstall functionality. Some mods modify various data files that are part of the core game, but if there isn't a project/package installed before that mod that also contains those files, then the files are deleted instead of restored. Having the ESMs and BSAs in the core package might be overkill, but it's better to be safe than sorry.
The current version of Wrye Bash has been improved upon for years unlike Wrye mash. It currently still has this very same issue. This is a practice by some but not common enough.
After you restart Mash, 'Refresh Data' (Installers), or perform a 'Full Refresh' (Installers), the Installers tab detects the core ESMs (Morrowind.esm, Tribunal.esm, and Bloodmoon.esm) and their BSAs as missing, if they're part of a project/package and that project/package is installed.
I created a TES3 package with Morrowind, Tribunal, and Bloodmoon as sub-packages, with the base contents of the 'Data Files' directory after installing the game and its EPs. This package is for full install/uninstall functionality. Some mods modify various data files that are part of the core game, but if there isn't a project/package installed before that mod that also contains those files, then the files are deleted instead of restored. Having the ESMs and BSAs in the core package might be overkill, but it's better to be safe than sorry.