Zombies burn during day time underground

This happens on our new v1.14 survial server. The world was generated with this server.

I've found a mob spawner in this world and build around it a 22 block drop shaft down to level 12 . It worked quite well so far. After logging out and loggin in the next day, All the sudden all dropped zombies went up in flames shortly after they landed.

It looked like the surrounded blocks were light up by daylight, eventhough there was no sky access. During night time they did not burn.

I removed some blocks and placed some to update the lighting and then they didn't burn anymore.

I have no evidence or screenshot of it sadly, however I might have this errornous state in a backup still if you require the world data.