Bitcoin core v0.19.0.1 automatically prunes. I keep loosing the blockchain. How can I stop this?
Start bitcoin-qt with the -choosedatadir option. You'll get first startup dialog again. Uncheck pruning there if it is checked and make sure the datadir is pointed to where you want it to point to. Once you click OK, double check the settings that pruning is disabled. Then check for a bitcoin.conf file in your datadir. If it is not there, you are fine. If it is, make sure it does not have have a prune= line.

Bitcoin Core does not prune by default, but recent changes to the Choose a Data Directory dialog may automatically enable pruning.