Already visited chunks in the Nether are not loaded/displayed (see screenshot). Leaving the nether and coming back into the nether sometimes may load the chunks, sometimes not.

If chunks not loaded/displayed the player can walk into the chunks as normal. Also mobs are spawned and visible as normal, but no blocks are displayed.

When the user walks into the unloaded/not displayed chunk, it does not trigger a load of the chunk, nor other chunks.

It seems the nether is just cut off at a certain line.

Problems started after update to 1.14.60

Logging off from the server and logging on again will load the chunks.