This issue I am currently running into suddenly after 6 months of using this command to create my adventure map for my friends is a bit hard to explain, so I will explain my process first:

1. I create a 4x4 of blocks of my choosing to use as a ground type I would like.
2. I place down a repeating command block with the command execute @a[m=c,tag=brush] ^ ^ ^10 clone xyz uvw ~-3 ~-2 ~-3 masked normal.
3. I will write down the coordinates xyz and uvw of the 4x4 I created.
4. I will activate the repeating command block with a redstone block and now my "paintbrush" is ready to use whenever I add the tag (brush) to myself as long as I am within the range of the 4x4 and command block.
5. If I go ahead and add the tag to myself it works fine, until...
6. When I'm working a little too far from the repeating command block it will not longer be able to execute the command. (Side note: My render distance is set to 40 so the 4x4 is still within loaded chunks.)
7. Normally to extend my range I would place another repeating command block with the same command and coordinates, and be on my merry way creating landscapes. However now the secondary repeating command block will fail to execute even when I have the tag activated. I even tried to go back and remove the first command block to see if they interfered. However the secondary command block still would not execute and where the first command block was, was still executing the command as if it was still their.

The only workaround I found was just creating a new 4x4 every time I needed to extend my range. However because of how short the range is on the command blocks this has put a huge delay on my project and I now have no motivation to finishing the map....