Hi ! This is an issue that has been occurring for several Plasma versions. I double checked recently with both the latest KDE Neon iso + updated and the Manjaro KDE iso.

Short form : When removed for the "blacklist", devices are not indexed unless indexation is disabled then re-enabled

Long form : When one installs a KDE distribution on a machine that has a separate data partition, this partition is usually automatically put in the Baloo indexer blacklist and is thus not indexed. Removing it (from the KDE "search" configuration panel) from the blacklist doesn't get it indexed.

To get this un-blacklisted partition indexed, one has to disable then re-enable the indexer. Just rebooting doesn't work.

Steps to reproduce :

1) install KDE Neon (or Manjaro KDE). In the installation wizard, choose a distinct ext4 data partition that is mounted under /data.
2) reboot, update the system
3) open the KDE control panel -> desktop search -> remove /data from the blacklist
4) wait a little, and notice 1) baloo is idle 2) files from /data are not indexed
5) reboot 
6) repeat 4
7) open the control panel again. Disable indexing completely. Re-enable it.
8) notice that the baloo process is not idle anymore and that files from /data are indeed being indexed

Reproducibility : always
Plasma 5.12.1
KDE Neon or Manjaro KDE