Two things can cause this… slipping and material surface not being level/flat.
It’s hard to tell from the pics because it’s apparent that you ran several test runs, so I can’t be sure.
If, for example, the right hand pic was all one carve, then I would say your material was not level to the machine. This is assuming it finished one line before moving on to the next. Either your wasteboard is not leveled properly, the material was thinner on the right than on the left, or the clamping allowed the material to lift on the left side during the carve. It’s interesting thought that the Hebrews 4:12 came out fine while the line above it did not. To test to see if the problem, zero your v-bit on the right side and manually jog it to the left side… does the v-bit point drag into the material as you progress from right to left? If yes, then the surface of the material is not level
If, however, you ran each line as a test, and zero’d the v-bit each time before running the next line, then you have a slipping belt. An easy test to confirm this is to carve a line, then send the bit back to the original zero, and come down 6mm so you’re at what the machine thinks is 0,0,0. If the bit is not zero’d properly to the material, then you know it gained height as it went along… and you need to follow the slippage troubleshooting Will posted.
Edit: I just realized you’d already resolved your issue… one thing I would suggest measure that board to see if it really is thinner on one side than the other. If it is not, and you already checked your wasteboard to ensure it’s level, then your clamping is allowing the material to lift on the left side… those smooth MDF brackets may be the issue. Try gluing some thin strips of high grit (220) sandpaper to the edges to give them some grip.