That is pretty weird. Which OS are you using?
Viewing post in Sealed Bite GameJam Version comments
https://www.reddit.com/r/godot/comments/9ljor3/psa_tilemap_flickering_possible_f...
This could help, however it seems like it doesn't work always. Having this problem in the editor was very annoying (it hasn't happened after I updated Godot). Anyway, I've enjoyed this game a lot so far!