SUMMARY
The tool to align the category of operations with their payees' always fills in the category of the first payee it found.

STEPS TO REPRODUCE
1. Have some payees with multiple different categories filled in
2. Have some operations without category filled in with payees that have different categories
3. Click on "Tools > Align the category of all single operations with the category of their payee"

OBSERVED RESULT
For any operation that matches a payee, the category filled in is the category of the first payee found. 

EXPECTED RESULT
For any operation that matches a payee, the category filled in is the category of its own payee. 

SOFTWARE/OS VERSIONS
Linux/KDE Plasma:  Skrooge version 2.30.0
(available in About System)
KDE Plasma Version: 5.27.8
KDE Frameworks Version: 5.107.0
Qt Version: 5.15.10

ADDITIONAL INFORMATION
This feature was introduced in commit 22a25c542f2502929a2cdc56d400763c2c18262d

The problem seems to be that the category of the first payee is remembered somehow. This even happens when you trigger the feature multiple times independently. The alternative reproduction for that is as follows:
1. Have some payees with multiple different categories filled in
2. Have some operations without category filled in with payees that have different categories
3. Select one such operation
4. Click on "Tools > Align the category of all selected single operations with the category of their payee" (the name of the menu item changes slightly when operations are selected)
5. The category of the operation is correctly updated with the category of the payee
6. Select another operation, one with a payee that has a different category than the previous one
7. Click on "Tools > Align the category of all selected single operations with the category of their payee" again
8. The category of the operation is set to the category of the first selected operation's payee, not this operation's payee.
9. You repeat steps 6-8 but it will always set the category to the first selected operation's payee.