Hello,
Are these the two drivers you are referring to?
https://igpmanager.com/app/d=driver&id=14380343
https://igpmanager.com/app/d=driver&id=12527005
When I took a look, one driver shows as Level 4 and the other, Level 5.
When you say their level goes down, is this their driver level, the "progress bar" on the training page, or specific attributes of the driver (eg. morale)?
Sometimes, a training session can result in so much driver improvement that the progress bar appears to go down without reason - but this is just indicative that your driver has seen an increase in one (or more) of their attributes. Once the progress bar reaches 100, it resets and a driver attribute will improve.
Next time you complete a training session with your driver(s), please can you take a screenshot both before and after so that we can take a look and diagnose the issue - the only attributes that should go down are weight and BMI (BMI will converge towards an optimal figure, indicated by it turning green).
I hope this helps. If it doesn't, please let me know and I'll try to provide additional assistance.