Sometimes, when I start the launcher in Offline Mode and launch a version, it says that the [version].json file is not found. So I have to go online and download it again, then make a duplicate and rename it. For example, I want to play 1.12 and the 1.12.json is still in

.minecraft/assets/indexes/

for both Windows and Linux.

Everything I say under this line are things that I both tried on W10H and U18.04

I tried open the launcher and the directory above side by side, the [version].json is still available. At the moment I told the launcher to launch the game, the [version].json get permanently deleted (not moved into trash) and the launcher announces that it cannot file such file in that folder.

And today, while I was playing 19w11a, the game crashed (this, which should have been fixed in 11b, but then still not) for the first time. I opened it again and it crashed for the second time, that means after the second launch the 1.14.json was still there. After that I add 19w09a to the launcher and tried to load my broken world in that version, the launcher announces that 1.14.json cannot be found. Then after failing launching 19w09a I tried to launch 19w11a. At this time that json file is no more there so I can play neither 09a nor 11a in Offline Mode

(The quality of the Internet at my place is so shitty that I can't even upload a photo)