I searched both here and the forum and couldn't find anything related so I hope this has not been mentioned before. Note that functionality is not impeded. The games works normally. Its just a cosmetic issue.

Test Condition:
Clean KSP 1.7.3
SSTU-0.11.48.160 and dependencies from SSTU zip
-force-glcore

First Test: without SSTU folder
Start career and see that all tech tree icons show correctly

Second Test: add SSTU folder
In the existing test career from above or a new career notice that the following tech tree nodes have parts but the icons are blank unless you hover over them and look at the popup.

1. Flight Control (Note that this one initially shows but after clicking through any of the following, icons will turn blank)
2. Landing
3. Advanced Landing
4. Heavy Landing

Example screenshot

KSP.log

Thanks for the report. Had noticed this in my recent career play-through as well, but kept forgetting to open a report about it. So now I can't forget about it :)

IIRC when I peeked at the logs it was complaining about a missing part/upgrade/icon when you click on the node in the editor (throws a null-ref), so this should be relatively easy to track down now that it is in the issue tracker.