I’m pretty sure - around 90% - that tabs are not allowed in a valid Tnsnames.ora file. They are one of many forbidden ones, like most unicode characters and those others with an ascii code less than 32 (space).
I’m not 100% certain of the tab character though, I’m well away from the docs, and my Tnsnames.ora validator program which you can find on github.com/NormanDunbar somewhere.
I wrote it a while back while suffering from a corrupt Tnsnames.ora which looked ok. It turned out to have unicode invisible characters in it due to someone copying and pasting from a Word document. This prevented some aliases from working, as you have found!
Norm. [ TeamT ].
Sent from my Android device with K-9 Mail. Please excuse my brevity.