Ah OK, I might have found the solution actually. Looks like there was a regression 2 days ago which was fixed yesterday but that fixed was merged a few hours after the last ./launcher rebuild app I did yesterday.
Have done another rebuild and it’s fixed. Might be worth leaving this report in place in case others have the same issue between now and the next time they rebuild. It would likely only affect users who were running a version of Data Explorer from Feb 7th/8th. I was just unlucky here, I installed the DE yesterday lunchtime (GMT).