Hi
First, please do not define attributes called VALUE. VALUE is a built-in attribute, the content of the marked text field:
So basically you created a recursive call with VALUE inside the value, and the additional attribute is kind of illegal. To be honest, I’m not 100% sure what LibrePCB does in this case – maybe everything is ok anyway but it’s still better not to do it. Just use MPN, DEVICE etc. within the value field but no manually added attribute as you don’t need it.
However, I think this was not the reason for your issue. Somehow it looks like your device (Q2) is not placed in your board, or am I wrong?
Any MPN or DEVICE attribute is indirectly related to a board (because they could differ between multiple boards) so the VALUE in the end is empty if the device is not on a board. If you export the BOM from the board editor (instead of the schematic), Q2 will be missing entirely. From the schematic editor, Q2 is there (because the device is in the schematic) but VALUE will be empty.
See these docs for more details how the system works: Assembly Data | LibrePCB Documentation
Btw, this is also unusual:
Since the second row doesn’t have “Mount” checked, it is basically useless (discarded). The device will appear in the BOM anyway because there is another row which does have it checked. It’s valid but unusual (you could do it to enter an alternative MPN but actually don’t want it in the BOM).