There are potentially two bugs in this topic, but Im unsure if the second is indeed a bug, or rather a (perhaps bugged) feature:
1) The "GNR 203 Class "Hawthorn 6ft-6in Single" 2-2-2" and its sister, the "GNR 223 Class 2-4-0" both needs the tender "GNR tender". Problem is, though, that the tender gets obsolete at 1860, while the locomotives are available after, 1862 and 1867 respectively. That means that there is a gap where they want a tender that cannot be bought anymore.
However:
2) When buying one of the locomotives, the obsolete tender gets bought anyway automatically and shows up nicely in the convoy! There is a consistency, though:
When building a convoy with this obsolete tender, and pressing copy convoy, an error pops up telling that you can't copy obsolete vehicles....
Thank you for the report - I have now corrected the inconsistency with the tender.