I encountered this problem when migrating my project from 5.4 to 5.5. Before that, I consistently migrated the project from version 4.24 to 5.4 through all intermediate versions, and everywhere the compatibility was complete, up to 5.5
The problem I have is that the inventory system, which took several months to develop, has become partially inoperative. The inventory is based on a structure that includes a number of variables of different types. The inventory itself works, items are added and removed from it. The subsystem for equipping items on a character has stopped working. It reads a string variable-identifier from the structure, and through Switch on String, a switch is made with the choice of the item to be equipped. In version 5.5, the string variable from the structure is not read.
Version 5.6 was released yesterday, I installed it, and found that equipping artifacts worked in it. But it has other problems - for example, from what I found, the names of save slots are not displayed, and the save date is displayed as January 1, 0000. It is likely that this error is also related to reading from structures.
It occurs mainly with structures created in previous versions of the engine, and manifests itself in child objects. Structures created in version 5.5 work fine.
I understand that 5.6 is still a Preview version. Let’s hope that the annoying errors will be fixed by the release.