To anyone else who may be having trouble with this now, you can delete these broken mobs by setting their {DeathTime} back to a value under 20 by targeting their UUID. Just look at them and if you type "/data merge entity a10d2397-619e-4194-9a51-a78dd41fef2c {DeathTime:19s}" it should autofill their UUID for you.
Ben Wilcox
To anyone else who may be having trouble with this now, you can delete these broken mobs by setting their {DeathTime} back to a value under 20 by targeting their UUID. Just look at them and if you type "/data merge entity a10d2397-619e-4194-9a51-a78dd41fef2c {DeathTime:19s}" it should autofill their UUID for you.