If for example:
TSB (0xA00A*) defines system TSB_TBRBridge

and TBR (0xA010*) defines system TTBTBR

and then a separate systemmap defined in TSB with extends="&unidAETheBackroads;" exists conditional topology link exists in TSB to create a gate between TSB_TBRBridge and TTBTBR, it will fail, because it has not created node TTBTBR yet, as it appears this is because TBR has a higher UNID number (0xA010 > 0xA00A). Doing the link on TBR's side works however.

xml
BUG1_TopologyA.xml
xml
BUG2_TopologyB.xml
wolfy 27 Apr 2025:

Turns out this is not an issue, there is an intended way to handle this using the <Uses ...> system which TBR does not (currently) use, 2 example mods that show how to handle this are attached.