Zend certified PHP/Magento developer

Customer group changes back to ‘general’ after visiting frontend – Magento 1.9

We have a storefront in Magento 1.9 that we only use for backend order entry for our largest clients. These clients are in customer group ‘MKB’

We also have our main webshop storefront where clients place their own orders. These clients have the customer group ‘general’

This is what’s happening:

  • I open a specific customer that is in the ‘general’ customer group
  • I switch their customer group to ‘MKB’ and save. Backend now shows ‘MKB’.
  • When i login on the main webshop storefront with this customer account, everything stays ok. Backend keeps showing ‘MKB’ customer group.
  • But as soon as i reach checkout (cart), the customer group changes back to ‘general’

What can i be doing wrong in configuration?

Let me know, thanks!