Not really a defect.

But the information says Platform is R 8.5.2.

That is wrong. A R 8.5.3 feature is used:  tagName for the panel control.

In the 8.5.2 Domino Designer you get the message:

This Custom Control was edited in Domino Designer 8.5.3 and uses features available only in that version.
Saving any changes could cause the page to no longer display at runtime.