That could be lots of things and, given the stress she's been through it's hard to tell and you wouldn't want to try to fix something that causes something else.
It could be either nutrient burn or lockout, or a temperature issue that caused a pH issue that caused a lockout. Now that your environment has stabilized with your new tent and light let's see if she settles in. But, quite an improvement over the past few days.
For now, since the rest of the plant looks much better I'd just keep an eye out to make sure it doesn't get worse or spread.