Same issue here as described by Jonathan and Alfred a.o.. Kstars/Ekos on a Raspberry Pi 4 / Astroberry upgraded to v.3.5.5 from previous versions with (regular) apt-get upgrades.

Kstar - Data - Download New data - Install [any catalog]  will download the corresponding .kscat file but it is not marked as installed in this window
nor listed in
Kstar - Data - Manage DSO Catalogs.

However, as described by others,  the .kscat file(s) are downloaded in ~/.local/share/kstars.
I tried using the "Import catalog" function of Kstars - Data - Manage DSO Catalogs, and selecting these .kcat files I did manage to install the catalogs.
(Import catalog did not let me browse into the .local folder, so I copied the files first to a reachable directory.)

They catalogs are then still not marked as installed in Data - Download new data, but I can now search again for these additional DSO objects with the find object function.

(I do sometimes have to type the full name though before some items are visible in "Find object" - not sure if this expected behaviour or also a new issue)
(For example, after installing the Sharpless catalog, typing "SH2 3" will list a range of entries starting with "Sh2 3" but "SH2 32"will only appear after I type the full entry)