Creative flat worlds missing chunks

Hello

The problem: Since the 1.2.13 update, both of my creative flat worlds (used for testing purposes so: simulation distance=4, always day, no weather, no experimental gameplay) have started up and loaded with massive amounts of land missing. Basically they have only generated land in areas I have already been - any chunks further out than that are just gone. - Please see screenshots.

I would expect the worlds to stretch out to infinity like they used to.

I do not know how to reproduce this since generating a new creative flat world seems to generate normally

Thank you for your time.