Nozzle Cleaning failed after the printbed goes all the way to the bottom to 338mm - firmware 6.5.2  

Nozzle Cleaning failed after the printbed goes all the way to the bottom to 338mm - firmware 6.5.2

Hi.

I wanted to let you guys know, that I belive I have found a bug in the 6.5.2 firmware for the Core One L. - 

The behavior is like this.

1. The printer was working fine, it finished a print, the bed moved down and I removed the print. -Everything normal. 
2. I selected to unload the filament, and while doing so, the bed moved  down again. 
3. I loaded the filament, and while doing so, the bed moved again down, so far it reached the bottom of the printer,and I heard that it skipped a few steps when it hit the bottom. - I thought nothing of it at that time.
4. I started a new print. It started to head the bed and when it was ready, it started the nozzle cleaning procidure. I then noticed that the nozzle drove into the print surface, and draged the nozzle to the left a few times, and then right and left a few times, ending with a nozzle cleaning error.  I chose to cancel the print.
5. I ran the Z alignment calibration . and started the print again. Now the nozzle cleaning completed normally, moveing the nozzle to the bed , lifting up, moving to the left, and down again , did this few times and finesed and then it ran the bed leveling and started to print normally. - for sake of testing, I stopped the print.
6. I then went in to the z axis movement menu, and moved the bed down to the bottom. At 337mm, the bed stopped moving, and skipped a step while reporting to be at 338mm.
7. I ran the print again, and during the nozzle cleaning, I was back at having the nozzle running into the bed and scraping it to the left. I canceld the process.
8. I ran z alignment calibration again.
9. I started a print, and nozzle cleaning process was working correctly and the issue was gone. 

I was repeatedly able to cycle trough this steps, and recreat the issue and fix it. - I was with Prusa support during this process, we tried several things, like lubricating the Z axis theded rods, losening the  trapezoidal nuts and retighen lightly. we aslo moved the heater wires away from the main wire  going to the budday board on the printhead as far as we could. We tried firmware 6.5.1 - same issue.

I recorded several videos of the process and shared with support, and here is a video of the issue being created by lowering the bed, getting the issue, and how to get it to work again. https://www.iskort.is/download/prusa/video4.mp4

We ended the support session on agreeing it must be a bug in the firmware, allowing me to move to 338mm and skipping steps, when it really should not go more than 336mm. Everything on the session will be shared with the dev team , so they can work on the issue and hopefully then can resolve it by changing the max movement to 336mm and issue a fixed firmware soon.

The support rep was super nice and helpful during this process, guided my though everything he wanted me to test, but it took a while to get to this point. At the end, he offered to  send me a new print sheet and nozzle, - and I accepted that 🙂

cheers.